I just spent like 4 hours thinking up how to make Undertale better by giving you the option to kill Asriel in the pacifist ending to betrayal kill all the main characters at the same time and based on if you have the genocide flag triggered it would cause Frisk and Chara to both talk to you and beg you to reset and if you don't for half an hour then they realize you don't want to reset and you can make them walk through the door where the loss of all their friends simultaneously just when the hope of saving everyone was in their grasp causes them to become suicidal and lose their determination to live so it shows the battle screen with them standing together around their shared soul and it shudders and cracks and they fade away and the game become unplayable and unresetable unless you either reset it to factory state or buy something from toby that lets you keep playing except this time flowey knows something is wrong and is terrified of you/frisk/chara and the tutorial fight with flowey becomes an actual difficult boss fight and then you can choose to kill him or not and the pacifist and genocide routes are impossible to get and if you left flowey alive then he talks to you at the end saying stuff based on how you acted and saying that something feels wrong and he doesn't understand why he was scared of you when he doesn't have feelings and then he says stuff related to how you played through and regardless if you kill him or not you can either choose to go aboveground or stay underground for your ending and if you go aboveground it says you tried to get back into your normal life again but memories of the monsters make you think that maybe things should have gone differently and you can either reset or not and if you don't it says "you sighed and decided that they were down their for a reason, whatever reason that is" and then you have to close and open again like the genocide route in order to get the same option and if you reset it's like "You're DETERMINED to learn more about the monsters" and it resets or if you stay underground the ending is based on your LV if it's 10 or higher or you kill Asgore (which would auto make you 10) then you move in with Toriel and it's like "oh you have a good life with Toriel, she's a great mom and makes great pies and teaches you a lot but you feel like the ruins are a bit restrictive" and shit and then it's like "you get the feeling that maybe things could've gone differently if you tried." and if you don't reset it's like "you sighed and decide that you're happy with your life with Toriel" or if you reset it's like "You're DETERMINED to finish what you started." implying the genocide route since you're high LV or if you're under LV 10 you stay with Asgore and it's basically the same but he's a great dad and the castle is nice but he really regrets his past actions towards humans and it's like "you get the feeling that you didn't try as hard as you could have" and if you don't reset it's like "you sighed and decide you're happy with your life with Asgore" or if you do reset it's like "You're DETERMINED to do better." implying the pacifist route because low level or if you end at LV 0 Asgore and Toriel get back together and you live with both and it's like they're great parents and you explore the underground in peace and any future humans are treated as friends instead and you love your life but you feel like you could've done even better and feel bad that the monsters are stuck undergound and if you don't reset it's like "you sighed and decided that your life is perfect as it is" or if you do it's like "You're DETERMINED to make everyone as happy as you are." and depending on your LV Flowey will say different stuff at the end if you spared him at the beginning like "I don't know why I was scared of you" 10+"Even though you killed so many monsters. After all, I've killed them all too, and you didn't kill me... I don't understand you. In a way you kind of remind me of my old friend..." -10"You only killed monsters in self defense after all. Which is weird that you spared me, since I was the first to attack you... I don't really understand you. In a sense, I guess that reminds me of my old friend..." 0"Especially since you wouldn't hurt a damn fly. Maybe all your hatred is directed at humans, not monsters? That reminds me of my old friend..." And then he'd say that something feels weird and he can't figure out what it is and he'll ask you to reset and help figure out what it is, so if you do a different ending LV he'll make that comment then say "thanks for actually trying to help. We should keep trying new approaches." and ask you to reset and once you do all three he'll suggest trying something new by either killing everyone or befriending everyone which will allow you to do the special true pacifist events which will lead to flowey saying "looks like that wasn't it. Let's keep trying." and then if you do genocide sans won't fight you he'll just judge you like a neutral route and flowey will suggest "maybe you need to kill everyone, me included. Let's fight again when you reset." because he won't have been fighting after the first time you spare him and once you kill him then genocide sans will still just judge you like normal but kill asgore will put you to 20 and you'll see chara talking to you and he'll say "I can't seem to destroy this wretched world. I should have the power to. Something is stopping me. We need to reset and find out what." and if you refuse he'll take Frisk's/your soul like in genocide or if you agree then he won't and either way now Chara will be saying things on your behalf and explain what happened to flowey at the beginning and flowey will suggest retrying it and the mercy tab won't have spare on it anymore and when you get to the end chara will either give you your soul back or take it, try to destroy the world and fail, then give it back and then you'll be forced to reset and Flowey will realize that you never killed sans so that might be it because it seems like it doesn't have to do with who you befriend or kill so you'll have to genocide and sans will do the neutral judging again with the question "do you think even the worst person can change" and shit but you'll be given the option to say "yes" or "no" and if you go "yes" he'll have his eyes closed and be like "... heh." and then walk past you and leave, then when you get to the end flowey will say he followed him and he's just staring at a picture in his lab, or if you say "no" he'll do that face where he's looking up to the corner like he's happy and say "yeah, I sort of figured you'd think that" and then it'll pan back to you and he'll be gone and flowey at the end will say he followed him and he's staring at a picture in his lab and once you've done both chara at the beginning will tell flowey they know how to fight sans and tell flowey to "get the idiot hostage" and then you'll do the genocide with no option to spare and at the end sans will be like "You kidnapped my brother... You must not have been able to get me to fight before, huh? What makes you think I'll fight when he's your hostage when I wouldn't fight you if he were dead? If you want me to fight you, that's all the reason I need not to. I don't plan to fight a fight I can't win. I know you'll just keep resetting until you eventually beat me. So I won't bother." Then papyrus comes out and flowey convinced him that sans is an impostor who killed the real sans and you get caught in the crossfire and after like 15 rounds of not being able to attack because it says "you shouldn't get involved yet" where if you don't have the locket equipped papyrus will oneshot you and checking him says "100 attack 100 defense, he's stronger than you'd thing, it looks like he's serious about killing sans." and checking sans gives the usual genocide description but "refuses to fight" instead of "can't dodge forever" and after like 15 rounds suddenly sans will attack with blasters and be like "figured I could catch you off guard. guess not." and then all the sudden he's sweating from papyrus' attacks and you have to dodge both their attacks at the same time and it's harder than the sans fight but after 5 rounds you get the message "now's your chance" and if you fight sans dodges and if you fight 2 more times you finally hit him and hit him repeatedly automatically like with flowey on genocide and it does more damage each time like adds a 9 at the end each hit and hits 8 times and then it shows sans' soul breaking like other bosses and then flowey surrounds papyrus with pellets and kills him and then you all say "so it wasn't killing sans, then" and decide to do the neutral route and end up with the flowey fight where he automatically kills you and then restores you saying that just made the feeling worse and then you beat him like a neutral route and he fights all out saying "fighting you is fun, chara" as his reason and when you win you kill him automatically then he's suddenly restored and chara tells him it didn't work, then flowey suggests trying to befriend everyone again and you're forced to do the true pacifist route then the asriel fight and he auto kills you then you're restored and he says it just made it worse again, but that he and chara should have some fun fighting all out and so you fight him again and can't hurt him at all so you have to do the pacifist thing and save everyone, then you realize you can't get out the door to the surface for some reason, so you reload automatically to the fight and tell asriel it and then chara says "one last thing to try" and you fuse with him, causing you to wake up at the beginning again but now frisk's sprite has a multicolored shirt and he has a smile but his usual eyes and it starts with ghosts of all the main cast wondering what's going on and chara and young asriel explaining that they could save and reset and the weird feelings, and sans says that he noticed that the timelines had done something impossible and so you're able to wander around the world with no random encounters. You can find So Sorry and asriel is like "we didn't kill him..." and chara's like "he's doesn't feel important" and alphys is like "he's clearly non-canon" and then eventually by exploring you have to figure out to go to the door in snowdin and click it to open it and sans will find the picture from his lab their and wonder how it got there, then you have to figure out to go to temmie village where you find the strange badge from his lab and he wonders how it got their too and suggests going back to his lab, so you auto do and use his power to teleport the machine to alphys' lab at her request and the two of them start science talk in the background and undyne is like "so while the nerds do their nerd talk why don't you tell us why we're all fused together" so it's like "you explain what's been going on to everyone else" and the screen goes black for a second and suddenly the machine is hooked up in the underlab near that thing that looks like neutral boss flowey and everyone is like "well it's really disgusting that you played with our lives like that but I guess if there's really something wrong it's better that you at least tried everything you could instead of not trying and it maybe being the problem" and sans basically says "yeah at least you tried everything by the way the machine is set up now it broke about when frisk fell but the results show that something totally impossible happened and then all the sudden timelines started warping all over, probably from you guys trying to figure out what's wrong." and chara's like "so none of you guys had the feeling of wrongness?" and they're all like "not until we woke up combined like this" and alphys is like "it's probably because you three are the only ones who can manipulate the timelines" and then you're able to wander again and eventually you need to figure out to take the hooded boat to each place from each place, the first time will change the color of it when you get their and the second time will make it static-y and once all three are like that and someone will comment on each change as they happen then sans will suggest going back to alphys' lab and when you get there you go through a weird door. If you check your stats during this time you'll have "Endless" attack and defense and 0 exp but 100 lv and your equipment will be 7 souls for weapon and armor. When you go through the weird door you'll end up at gaster and he'll say stuff that sans'll translate telling them how about chara's "partner" an entity that's guided frisk and chara since they entered the underground and that he's watched countless of these entities guide frisk and by extension chara and they always did one of three things, either totally peaceful, totally genocidal, or a mixture where they'd reset but that the one currently guiding them did something new and terrible and he'll talk about how it's impossible to know the entity's intentions or desires or reasoning and he'll say that he's locked out the entity's control so everyone can talk without being influenced and he'll muse on the possible reasonings for the ending where frisk and chara kill themselves and he'll eventually reveal that that's what happened and that this entity overpowered frisk and chara's shared will and DETERMINATION to save their mutual friends and used that very power to kill them all by killing asriel while they were all fused with it, then forcing them to leave, leaving the rest of the monsters trapped underground and how this shattered frisk and chara's determination and made them try to fall into ebott again which killed them but they had no will to live so their soul was destroyed and thus the entity's influence on their world was broken, and that he doesn't know how the entity managed to recreate the world and he'll reveal that he knew about how chara would destroy the world if frisk was lead down the path of violence which showed chara, still angry at the world, the fun of killing monsters as well as humans whereas other times chara would learn from frisk that there are still humans who are worth saving and that monsters still deserve mercy leading to him deciding humanity deserves a second chance if frisk is led down the path of peace, and that he knows that this entity didn't give chara frisk's soul via the route of violence because doing so causes a different ending upon betraying asriel where chara is happily surprised by his partner and that he would therefor being more than willing to reset when the entity returned so they could have more fun together, but doesn't know what else this entity did because all gaster's knowledge on this particular one's actions and influence before that final betrayal kill disappeared when this world was remade, which is also why he can now interact with it and then he'll comment on how the entity is determined since they've guided the three children through every possible path they could take in order to help them find the problem they could feel, then he'll once again state the potential reasons for the entity's actions and ask the children if, "as sans has asked in countless timelines" they believe that even the worst person can change, stating that the question now has new light on it given what they just learned about this entity. Before they answer he states that as the children can attest, being able to go through the same events over and over can make them boring and make one want to find new things and possibilities and that asriel especially knows how that's influenced by not having a connection with the world or people being toyed with since he didn't have a soul at the time and the entity is clearly not from their world. All three say yes and then gaster states that he hopes the entity isn't ONLY acting on curiosity because he's going to ask some questions and he wants honest answers, stating that he'll do everything in his power to placate the entity based on these answers, and saying that what he does in response will be totally permanent, the entity would have to destroy time and rebuild it from scratch to return here and if it both had the power and desire to do that just to see the alternate possibilities of what gaster will do in response then the entity's determination puts that of everyone there combined to shame. He then asks basically what were your reasons, "curiosity, malice, boredom, to save the two who couldn't be saved if the entity went along with the children's' wishes, for a challenge, etc." and you can say basically say yes or no to each and what you answer will determine what he does. Like saying curiosity will have him say he understands that but cautions that curiosity can be dangerous and that even those of another world should understand, like monsters and humans do, that lives should not be taken lightly, nor taken lightly, and if you say malice you get another question of if you wanted to see hurt if it was just unavoidable but you attribute it to yourself anyway, boredom he says that it helps him understand the entity's mentality but recautions that lives are not supposed to be toys, no matter how powerful you are, if you say to try to find a way to save the two left behind he asks further if you only cared about one or both, and that'll affect the ending he provides for you, if you say for a challenge and said yes to malice he asks if the malice was a side effect of the desire for a challenge, if you say yes he'll say he understands since pushing people by hurting them can bring out their power, and if you say no he'll question further if the challenge is a side effect of the malice, saying yes affects the ending he provides and saying no makes him say that perhaps he simply can't understand the entity's reasoning. Etc. He'll ask you to save, and won't let you continue until you use the save point, saying that he wants you to be able to return here to see the ending that you seem to want without having to play with anyone's lives anymore, and if you gave your reasoning as wanting a challenge he'll fight you in a near impossible fight where he's very gracious and starts by saying that he anticipates both him and you tasting defeat many times over, then he'll separate everyone so it's just frisk and chara fighting him saying that it wouldn't be challenging otherwise, though if you gave malice but not challenge as a reason he'll fight you where you're way overpowered and can just wail on him as much as you want, then after the fight which you'll auto be brought to the beginning of on loading instead of your save "for convience" and with sans translating everything he says before the fight but not during or after the fight, with gaster basically saying "well fought" for early defeats or "that was quite the challenge for me as well" for fights that go further and before the fight basically saying "It seems it was my win" or "That was impressive." and then giving the number of losses you have and it'll just be a counter that goes on endlessly unlike sans so you'll FUCKING KNOW when it takes you 85919591087593573597 tries to beat him with his flipping perspecive and using all colors abilities and mixing colors effects and turning your soul black making you have to fight without seeing where you are or turning you white and his attacks black so you have to dodge based on sound ques, or if you didn't give challenge or malice as a reason you'll get an ending based on your answers, and if you gave the reason to save the two who couldn't be saved no matter what, he'll have created new souls for Chara and Asriel and give you the pacifist ending but now with them in it, everyone being happy and nobody being left behind. Then it'll autosave and if you load it'll just be gaster and then sans'll teleport in to translate and it'll basically be "do you want to see the ending again or fight gaster?" and that'll be totally unremovable without reinstalling undertale, doing the asriel betray kill, then going through all the resets to get there again which is what gaster meant by "destroying time and rebuilding it from scratch."
In short, you'd get to learn all the unknown secrets of undertale, including about gaster and sans' lab and picture and badge, but only if you're literally more evil than genocide route chara. Because not even he would think of befriending everyone in the underground, becoming close friends with all these monsters, making every monster rely on and trust in you to free them only for you to turn around at the last second and slaughter all of your friends simultaneously while also leaving all the other monsters stuck in the underground now without their king or any monsters who would normally take his place with no idea what happened to you or any of them, and you'd have to do it without having done the genocide route otherwise chara will basically fall in love with you for it.
Like there are literally people so attached to the main cast that they REFUSE to do the genocide route and toby fucking knows it and has flowey call them out on it, imagine if all the secrets to the game and sans and gaster and everything else were revealed but only if you actually thought to be more evil than genocide chara. And actually attacking during the fight would require a minigame harder than the sans fight since you're literally fighting chara and frisk's combined determination. Failure would show "You stood there, unmoving." That way asriel wouldn't know you're trying to actually kill him, though he'll have comments based on previous routes before resetting into the pacifist route about how he wonders if you'll fight back against him even though it would be pointless.
