If I were to GM (and, basically, that's the only way it would work), it would be one of Palladium's games, probably Heroes Unlimited, Nightbane, or Rifts. Combat is much simpler in a webgame like this, so Palladium's wacky rules wouldn't get in the way...I might even use the new house rules I came up with by stealing Inquisitor's damage system.
If we play HU (and that's my main choice, because I'm on a comics kick right now), I would try to keep everybody on a Mutant/Experiment theme, sort of like X-Men, just so I could take advantage of their new supplement "Mutant Underground."
If we play Rifts, I'd keep it simple (none of this "anything goes" character variety that so often derails Rifts games) and make heavy use of North America and the Burbs--again, because they just put out an adventure supplement that I could use. Alternately we could do Rifts Phase World, which is a big space fantasy, because they just released a great supplement for that.
There are no recent supplements for Nightbane, but I've always been interested in a historical campaign and this might be the opportunity. Nightbane are monstrous defenders of humanity who happen to be immortal (sort of like vampires or Highlanders). It would be fun to do a campaign where we start in ancient egypt (or something) and then each adventure takes place in a new historical setting: the Roman Empire, barbarian Europe, exploration of the New World, Pirates, American Revolution, World War 1 or 2, etc. This could be weird, and you'd have to forgive my occasional historical inaccuracy, but it might be fun.
So those are my ideas, and I'm definitely open to more. I appreciate Saint's advice, and I think this could be fun.