Fandom: Favorite Alternate Universes, Fusions, and Tropes

I'm not doing very well; September is a rough money in my family, and I've been struggling with the bipolar all year anyway. However, Trick or Treat sign-ups start soon, and I can't wait. I'm working out what I want to request and what prompts I want to leave for them, which made me think I'd love to hear from y'all about things you like in stories.

In particular, I'd love to know your favorite alternate universes, fusions, and tropes.

Some of mine, to start us off: werewolf AUs, monster hunting AUs, enemies to friends (to lovers), friends to lovers, IN SPACE AUs, Everyone is a Slayer Now BTVS fusions.

What are yours?