I would love the opportunity to play:
Warhammer Fantasy Roleplay - a grim, gritty adventure path. I think I would be tempted to play a rat-catcher.
Pathfinder - as soon as it is released, although chances are I will end up GMing a game rather than getting to play in one
Bloodbowl - and I am the Middenheim Marauders
Spelljammer - probably best in 3.5e or Pathfinder, but I would even play it in 2e if I had to.
Midnight - okay, another setting, but I would love the chance to play a game in this setting
Dark Heresy - an amazing RPG book to read through, and I am sure it would be fun to play with a mature GM
Phew ... that is enough for now. Sad reality is that I will almost certainly not get a chance to play any of these (although will GM Pathfinder)