No, anything goes in fanfic. I used to only read erotic ones when I started but so many of them started to feel very samey, like you could slot any characters from any any fandom into the story.
Now I prefer the type of stories that you're describing when you see small interactions or a peek behind the curtain. I especially like fics where the author does something completely different than everyone else.
Stories with pairings are always more read but there is an audience for everything.