So, you have all these game ideas that you want to run (and lots more now, thanks to Attacks) but there just isn't enough time to cram them all in. After all, we do have lives, despite being gamers. We have wives, girlfriends... or failing those, Grand Theft Auto 3. We just can't play rpgs seven nights a week, no matter how much we want to. What to do?
Here's my idea. Everyone generates characters for session number one. Make it something simple and direct, like Feng Shui, or BESM. You play a session of whatever: cowboys in space, giant robot teen angst, mercenaries in ancient Rome, etc. Now here's the kicker: Each and every session, the genre changes completely. You keep the "same" characters as the story progresses, but those characters are interpreted differently each time. They're translated into the new genre as closely as possible. The narrative thread continues, just mutated to fit the current style and place. You could come up with some sort of dimension-hopping conceit if you really wanted to, but I recommend just ignoring the changes as far as in-game knowledge goes.
Today, samurai warriors fighting the undead at the Shogun's palace. Tomorrow, space pirates trapped on the planet of the atomic zombies. Now that I think about it, a super-generic system like Fudge would probably work best. Keep the system stuff real simple, with stats like Fighting and Smarts just so you don't have to do lots of conversion work each time. The players should get a week's notice of the upcoming genre so they can re-work their characters. If the group is really digging a certain style, you could stay with it for a few sessions before moving on.