I don't typically have [i]one[/i] specific outline but some general things that are always a "yes" for me:
-LOTS of lore/worldbuilding, regardless of individual post lengths. I [i]love[/i] going into what makes the characters and the world they live in tick
-Cryptids/creatures of some sort, whether they're mains or just side characters. I'm always for having a Fucked Up Little Weirdo (affectionate) or two
-For sci-fi specifically, I'm slightly a sucker for the subtle cosmic/lovecraftian/eldritch horror. Make the characters an unwitting pawn in the game of some long-lost god or gods. make the magic system a little Fucked Up And Weird etc etc 

....honestly any of the above and you've probably got me LMAO