In related news, writing a synopsis of the story that's been bouncing around in my head lately. I never get farther than that, though, because these things are always based on other people's characters and mostly just a backdrop for angst-ridden interior monologue.
Basically it's not "take these characters from an existing thing and porn them" but "take these characters from an existing thing, with another character from an existing thing, and put them in basically the plot from 2 other things mixed together". It still feels like plagiarism, though, which is why none of these ever get past the synopsis/notes stage.
Though maybe this sort of thing is how Fifty Shades originally got written? My brain has way less porn than that, though. And way healthier relationships (not totally healthy though, because humans).