seems like at least a rotation is in order. Restricting to genres will also make it easier for people to vote. When the choice is between Final Fantasy XIII, Space Harrier, and Super Mario Bros. 64, people are gonna have a tendency to vote for which *genre* they want to play, not which game. I voted against Counterfeit Monkey cuz I didn't want to play a damn adventure game, but if it was Adventure Month, I'd at least have had a chance to pull for an adventure I'd have preferred.
On top of just genres, you could also do theme months: Turbo CD games, Mizuguchi games, etc.