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.
Rewrites of early episodes (usually Season 1) to include an OC, or in rarer cases a canon character who wasn't present in the original.
Rewrites 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.
Changing the order Slayers were called. For example, Faith becoming a Slayer before Buffy.
Road Trip stories that show much more happened on Xander's offscreen road trip than just his car breaking down and him stripping to pay for repairs. Often used to introduce crossovers.
Alternatively, someone joining Xander on his road trip. Cordelia being one of the more common ones.
Someone other than Xander being possessed by the Primal Hyena.
Buffy is sent back in time to circa 1880, and meets either human William Pratt or vampire William the Bloody. Almost always a straight Buffy/Spike pairing, though the plot can be used to set up Buffy/Angelus as well. Or both.
It's to be mentioned that, even when it is not the whole (or the most important) part of the plot, there are many Buffy fanfiction stories where a character fires off a lengthy What the Hell, Hero? speech at what the author perceives were serious flaws of the character or the canon Buffy series plot.
The organization of a "Slayer Academy" post-Season Seven. Varying from military-style boot camp to "sorority for superpowered girls" to a Hogwarts-style experience (sometimes it even is a course in Hogwarts).
Buffy takes the "General" part of her role during Season Seven to its more literal extreme: in one way or another, the military starts to employ Slayers for anti-supernatural Spec-Ops work. Buffy is either just the first, the leader, or pretty much The Only One. Crossover with military-themed shows (like the Stargate-verse or NCIS) is pretty probable under these circumstances.