1) Gender-switching. Love this. Have read what's out there. Want more.
2) Xander the Vampire Slayer. I've yet to really see something that stacks up against canon, something epic and convincing that uses the canon characters, but with twists and switches. Actually, at the moment, I can't remember seeing anything at all that uses this idea, but it's probably out there.
3) A crossover with one of my other fandoms. I've seen good Buffy/SG1, but not slash. Actually, even great gen would do for this category, and I have to admit, the X-factor is also not so big a factor here. Maybe someday I'll be kidnapped and forced to smoke crack and write that Fraser/Spike story. It would be my goal to terrorize the world with that story. ::dreamy look::
4) A Xander/Spike/Wes story. Plausible, happy, yes. I want to believe.
5) An all-human high-school AU. No. Really. I want someone to write a brilliant one; sharp and witty and canon in tone. I was actually hoarding that story idea for a while, like an unspent gold piece, because I thought I might write it. Probably not, though.
6) Some kind of slave story. A mute and animalistic Spike--or maybe one where the slave routine is just necessary and circumstantial. Either way, it's a romantic bond, and even, in a weird way, ennobling. More romantic than kinky. Passionate.
Looking at this list I see that...I'm amazingly predictable. And I feel like there were more stories I wanted to mention, but they've slipped out of my grasp for now, whatever they were.