The (in)famous Yet Another Halloween Fic sub-genre of fanfiction, in which Xander (and possibly other characters) choose a different outfit for the fateful Halloween when Ethan Rayne turned everyone into their costumes. Invariably, these will be the costumes of recognizable characters, particularly from anime.
What might be termed the "Slayer Speculation Fic", in which the famous, strong female character of the author's choice from another fandom is called as a Slayer by Willow's spell in the series finale.
Buffy and Firefly is such a Common Crossover that such stories have their own tropes (and considering the amount of 'Verse slang that pops up in the future-Slayer comic Fray, some fans wonder when Joss is going to declare that world to be Earth-That-Was and get it over with). For example:
Book is (or was) a Watcher. Jossed. Not that it needed to be...
River is a Slayer, and the Academy was a corrupt version of the Watcher's Council. (Well, more corrupt than usual.)
Rarely will more than one or two of the Scooby Gang feature, and it's usually Xander or Willow transported to the future/an alternate dimension via magical portal or suspended animation or an unaging Buffy and Faith
Xander is an enormous Badass, since he only has normal humans to fight, and has spent his life surviving against creatures at least twice as fast and strong as he is.
Mal will be mistaken for Caleb. Zoe and Jayne's resemblance to Jasmine and Hamilton may also be brought up, but this occurs less frequently.
Rewrite of season 6 to bring Faith back after Buffy's death at the end of season 5. Usually serves to set up a Buffy/Faith pairing after Buffy's resurrection, but there are other pairings and directions that the plot can take.