Well, I have at least come up with a plot for a Buffy game.
The other two GMs in the Urban Fantasy category are running The Dresden Files and Scion so that's a fair smattering of investigation, magic use and superheroic violence covered. So I'm going for a middle path between those two, with some extra jokes.
It would help if I had six strong PC concepts. I have a Slayer, a Watcher, a Sorcerer... maybe a Zeppo...
Probably another fighty type, though I'd like to avoid another vampire with a soul. Maybe a werewolf or a warrior demon or a ninja cyborg from the future or something.
Maybe a social-fu type, although they might get left behind in the really really really violent bit at the end.
I'm not sure about using the series cast or a new group yet. I lean towards originals, to give the players more space, especially not-fannish players. I may repeat the trick I did once with a Firefly oneshot and bring two sets of pregens. But that'll make it that bit more obvious that I'm really writing for the series cast and including understudies as original PCs.
Meanwhile, I also have several possible big-league apocalypses. Those aren't a problem. And getting there shouldn't be a major issue either. But what else is going on, in the characters' lives and loves and all that fun stuff?
As you can see, progress is being made. Of a sort.