I'm very fond of the original Star Wars RPG (though I like the dice mechanics of Edge of the Empire/Age of Rebellion/Force and Destiny, too); it just plays really, really well, and I'm interested to try to see how well it would work in PbP.
I'm interested in trying to facilitate a game of Fiasco (one of my favorite one-shot games), though it requires a lot of RP and patience from players on PbP.
I think both Gamma World (1st or 2nd edition) and Paranoia (1st or 2nd edition) would be fun to run; Paranoia is nice because it is generally clearly bounded. You can't really do a campaign of Paranoia unless you have people running a sequential series of clone families...which doesn't really interest me, as it feels like squeezing a square peg into a round hole.
I've toyed with the idea of running The Taint here on GP; the difficulty here is in not knowing the players very well outside of the forums, and so not having a good sense of what powers would be particularly satisfying/enjoyable. I'd probably use random tables, though that likely isn't as fun.
I also started adapting the classic diceless 80s RPG, Space Seekers (yes, the source of my username), to diced play here on GP. I'd love to do that, though it might take a lot of explaining for everybody.
Right now, though, I'm trying to take it slow and not over-commit to running a bunch of games at once. But it is tempting when I'm in so many games that aren't posting as fast as I would like. My temptation is to think: "Nothing's happening right now, so I should run another game. Or maybe, since I can't decide on just one, I'll run TWO MORE GAMES!"
So far I'm reminding myself to be patient, and these lulls happen naturally, and that impatience will come back to bite me HARD if I give in to it. Therefore, I'm not going to start another game just yet...but that is the list I'm considering for when I do.