I never did a genocide run either - I just watched youtube playthroughs, because I could never bring myself to kill Papyrus. Oddly enough, I'd still be perfectly happy with writing Chara. Possible spoilers: if you get all the way to the end of the Genocide run, your save file is forever marked. Even if you reset and do a Pacifist playthrough, Chara appears to haunt you at the very end - that's what I was thinking of when I suggested post-Pacifist.
I didn't know what Swaptale was until you mentioned it, but I Googled it and read the whole thing just now. It's great. If we want to roleplay something like it, though, we have to get all the characters together sooner. Pacifist!Frisk and Chara!Frisk hadn't even met by the time of the most recent page, and we wouldn't be able to interact before then.
If you just want Chara to have their own body, we could come up with an AU where that's true - maybe Frisk's determination revived them in their body, instead of as a malevolent spirit possessing Frisk? Or we could abandon Frisk and Chara altogether, and roleplay different characters.