This is a idea that I got from watching the show Dexter last night. It's not needed for you to have seen Dexter to fully get the idea however! If you have seen Dexter, then you basically don't need to read the rest. Just know that this plot is heavily based off of when Dexter found Lumen. Also know that we will not be playing the actual characters. Just our own who are slightly like that, but with our own twists and whatnot.

The male character is a serial killer who actually works for the cops as a blood spatter pattern expert. No one knows what he does at night, due to the fact that he lives by a certain code and rule number one, is "don't get caught." He's not like most serial killers, however. No, he only takes out the people that fit his "moral code". People that he believes have escaped the law, one way or another. He takes out those that have killed, raped or abused the innocent.

Despite the fact that he's extremely smart when it comes to both his day job and night job, he's not so great with people. As a young child he witnessed something horrific and it ended up shaping him as a person. He doesn't really know how to deal with people all too well. Emotions are something that he never really had and something he fully doesn't understand, either. He's practically a shell of a person, expect when it comes to the only thing he really has a connection with. Killing killers.

For whatever reason the male character has decided to kill a certain man. He's been playing with this idea for awhile now, but tonight is the night that he finally decides to put his plan into action. He sneaks into the mans house and does what he needs to do, but upon cleaning up, he discovers that he wasn't as alone as he originally thought. Through the small window on the door, he sees a blur of what appears to be a person. Quickly, he goes to find who saw him and plans on possibly killing the person, so they can't speak of what he's done and possibly get him caught.

Upon searching, he discovers that it's actually a girl. A girl that the now dead man has held hostage in his attic for months. This is where he decides to spare the girls life and actually try and help her, even though it could potently up getting him caught in the end. The girl isn't the only girl to have been held hostage however. There were actually 12 girls before her, however all of them were killed. The man that had been killed wasn't the only one who took part in these hostages either. They come to learn that there's actually five other ones and now they each must meet their fate.

(I'm just going to throw in a random name to help with the plot example. Feel free to name your character whatever you wish. This is just for the plot idea.)

Tyler never lived much of a normal life. His parents were always on the go, leaving him behind with whatever stranger they thought suitable to watch over him and the house. Once Tyler turned eighteen, they never came back. All Tyler had to know that they were still alive was their monthly letters, apologizing and giving false hope of their return home. Tyler still lives in the same home, though he has given up the idea of them returning one day. He often wishes for a better life, a normal life.

One night, while taking a walk in the nearby park something bright shines in the sky before it comes crashing to the ground below. Soon, Tyler finds himself face to face with an unconscious girl. Though, upon getting a closer look, he discovers that she is not just any girl. No, she has wings. Not knowing what else to do, he takes her home and waits for her recovery.

Once awake, the girl explains that she is actually an angel. A "pet-class" angel, meaning that she is now Tyler's pet. She wears a collar like thing around her neck with a long leash that Tyler can hold. Though the leash itself is capable of being invisible, the collar is not. She's capable of granting Tyler nearly any wish, dream or demand that he desires, however she is incapable of canceling it once it's been said. Even wishes that Tyler doesn't necessarily mean, are granted. Therefor, Tyler learns that he needs to be careful of what he says out loud.

The girl is almost robotic in the way she speaks and acts, unable to fully express emotions, though she is very protective of Tyler when he's in danger.
(That's basically all I have for this so far, so feel free to add in your own ideas to it! I'm also okay with switching the roles around if you want. Meaning, you playing a male angel, and me playing the human female.)

(This is based off of American Horror Story: Asylum, but it's okay if you haven't seen it!)

Two people find themselves locked inside possibly one of the worst asylums ever. The doctors here are twisted and still believe in using old practices of medicine. Basically anything goes here. Electroshock therapy, lobotomies, experiments and so much more. The "patients" here are kept in dark cells made of nothing, but concrete with a small, thin bed for them to sleep. They're allowed out of their rooms only three times a day. Once during the morning to shower with cold water, once during the afternoon for an hour to go to the rec room and one last time at night to do one of the many jobs given them. After that, they're locked back up in their rooms with nothing, but the cries of others to keep them company.

Most of the people here are in cells alone, however, on rare occurrences when the asylum has too many patients, they'll get a cellmate. This usually doesn't last for long as people die here nearly everyday and sooner or later, another room will be cleared for them.

Our characters will end up in a cell together at some point or another. This is where they decide that it's time to get out of this place and they come up with an idea of how to escape. However, when the day comes for them to finally get out of here, one of them is taken to a room of their own and now, the only time they can see one another is when they're allowed out of their cells. Will they be able to escape or will they end up just being yet another experiment for the twisted doctors?

Vampire/Human
Slave/Master
Homeless/Rich
Other Monster/Human
Teacher/Student
Angel/Demon

(OC only! Canons are allowed as sides though.)
Supernatural: Perhaps a monster and hunter end up working together? Maybe the hunter doesn't even know that they are actually working with a monster? Let me know if you want to come up with a plot for this.
True Blood
Harry Potter
Ouran High School Host Club
Pokemon
The Walking Dead