High-level 4e D&D. At the very least mid-Paragon tier; I'd consider going all the way to Epic. I wouldn't mind giving the system another spin, and a lot of my players like it, but I don't want to fiddle around with the first four levels again. I want to see what the system can really do. I'd like to try out my old "world ruled by rival factions of giants" idea, possibly with the players as freedom fighters against the giant oppression. But if I were to use as many kinds of giants as I imagine, and if any kind of combat came up with them, there'd be a lot of custom monster making, since there's only three kinds in the MM and one is kind of oddball. Not bad, just more work. I might end up just going with the "team of reckless archaeologists" idea; Indiana Jones seems like a good fit for the game's style. Either way, the real problem with this is that it'd awaken the temptation to buy more 4e crap, which is an urge I don't need.
Megadungeon. This should be ready for play by that time, so this'd probably be the lowest work option out of the things I'm considering. Some of the players are surprisingly enthusiastic about it, too. Unfortunately, a couple of them are flatly uninterested, and though I could probably convince them to try it I'd rather do something that everyone's excited about from the get-go. I'll probably break it out a couple of times over the summer for testing purposes; that's the nice thing about the megadungeon set-up, it's fairly accomodating of occasional but connected expeditions.
Vampire. Either Rockstar or Lawyers, Guns and Money, depending on what kinds of characters wanted to run. This is another option with a surprisingly high amount of player interest, even from the people who haven't wanted to play this for years. On the other hand, I'd really rather play Vampire than run it, and I'm not too sure about my groups willingness to "take it seriously."
Superheroes. Most likely Mutants & Masterminds, though one guy I know has HERO and likes it, so I'd be open to that option. I've had a game like this in the back of my mind for a long time, and it'd be nice to give the genre a shot for once. Unfortunately, though the group has several major superhero enthusiasts, it's got several people who actively dislike superheroes.
3.5 D&D Supplement Extravaganza. I have a bunch of d20 crap. My players have a bunch of d20 crap. We've never gotten to use all that d20 crap. I don't know if I'd want to go so far as to use absolutely everything the group owns, but I'd like to get a chance to use a lot of the material in Sandstorm, Tome of Magic, Book of Nine Swords, Expanded Psionics Handbook, Spell Compendium, and Magic Item Compendium that never quite saw play. So probably a weird fantasy/ancient world game, with a bunch of different competing orders -- Jedi-analogues/Bene Gesserit-types, ninjas, a bunch of different martial arts monks/sages -- and the players hook up because they all want to get the same cosmic doo-dad, and they figure they're better off working together until they get the chance to grab the thing for their order and run. Hilarity ensues.