I'm DYING to do a Black Butler plot right now. I haven't fully finished the anime yet, but I should be done in a few days, and I'm absolutely willing to start something up. I'm looking to play Sebastian, and I'm very open to how I play him, either in a M/m setting or a M/f one, modern day or not. The plots I have listed below are just loose ideas, but if you have any plots of your own, I'm more than willing to work with them as well. If you'd like to modify these plots or change anything about them, please let me know, as I'm very open to that.
1. Sebastian/OC: I would see this taking place a bit before Sebastian becomes involved with Ciel. The same general time period, give or take a couple hundred years. He would not have worked for Ciel yet, so this would be less canon than other plots. A young woman (or man) would be contracted to marry someone who they found repugnant. Their family has kept them sheltered for years yet has no particular concern over their child's happiness. All they want is for the marriage to go through in order to gain money and business connections. Your character is desperate to escape the marriage, or at the very least to have someone on their side, they attempt to kill themselves. In doing so, as their life slips away, they desperately wish for another future, a future in which they can do things themselves, a future in which they have someone whose only concern is to be loyal and keep them happy. Sebastian appears before them, offering himself in return for their soul, promising to serve them loyally and help them at every turn. From here, this could either go with your character beginning their own company, exploring the world, or entering into the marriage yet finding comfort and escape in Sebastian.
2. Sebastian/OC: Modern day. A young woman or man has grown up in abject poverty and is absolutely desperate to have a taste of the finer things in life. They hate the fact that people are able to look down on them because they have no money or connections. Their life has been hell to this day: every job they go for is given to someone who has a family connection, their rent is due, and their life is falling apart. They get into a bad situation (perhaps owing money to the mob, a car crash, suicide, etc.) and in doing so, Sebastian opens up a deal with them. He will give them everything they want, serving as a faithful personal assistant (or butler, however you want to title it), but they begin to realize after the initial rush of having money, they still don't feel like they measure up. Sebastian patiently waits to gather his prize, meanwhile. Either there can be a specific point at which he would collect it, or it can be at the point of their death.
3. Sebastian/OC or canon: This would be a more inherently sexual plot. In his few hours away from his master, while Ciel is sleeping, Sebastian seeks out a little physical relief. This could either be with one of the characters in the house (Baldo, Finnian, Mey-Rin, etc.) or someone outside (Lau, Abberline, Will, Grell, Undertaker, OC). With an OC, it could either be with a girl he happens to meet while strolling outside or he could specifically go out to a whorehouse to find a bit of pleasure. This is a looser idea, but very open to collaboration with a partner.
4. Sebastian/Ash (or Will, Undertaker, Grell, or any other reaper, demon, or angel): So this one is obviously a loose one, but generally I'd like to play him across from someone who is non-human. With Ash I imagine it would be a purely physical giving in, but with any other non-human character I'd be open to how things happened. With Grell I would imagine that Grell eventually wears him down and pounces on him after he is particularly sexually frustrated by something Ciel has done.
5. Sebastian/Ciel: C'mon, you guys had to know this one was going to be in here. Since I'm not done with the series yet (though hopefully will be in a few days), this would either have to be one set just after the first season of the anime but with Ciel aged up a bit, or else we'd have to wait on things until I finish. Definitely open this pairing, though, so if you're interested, please reach out!