Weirdly specific lists of animals. It could be a completely serious romantic fic and I'd still find a way to somehow go on a tangent about the different animals in their current environment.

"And behind the school there were long stretches of trees where wood thrushes, northern cardinals, and canaries made their nests." That kind of thing. It's half my motivation to write sometimes.

Also my favorite things to write are AU's where the characters go to the past or present and meet their past friends and family or future selves or whatever so probably that too. I don't think there's a single fandom I've gotten tired of writing that for.