Some brainstormed ideas:
a dance game:
A game that tries to teach you several different solo performances. Grades are given based on movement accuracy, beats, and so on. Bonus to this would be that it allows you to create "canned" smaller parts of a routine, which you can call up in the free-style portion of the game.
medieval melee:
featuring realistic combat engine, for the hat trick, allow online connection to play against other players. because of the lack of force feedback, the game will still need to feature some level of a hit-stun. Like SimuLord, these will require some subject matter experts. In this case, I'd pick people from ARMA.
Orchestra
IF we can do online multiplayer with this, this game can be even better. The starter of this is that each player can play as an orchestra conductor, conducting music and based on his performance and accuracy, you have certain performance. With online, we can also build roles for other players who want to play different instruments. Obviously, we'd have to simplify the mechanics of playing music since 1. we don't want to force people to have taken 12 years of cello just to play the game and 2. I'm not sure if the kinect can actually handle detecting very small minute movements like finger positioning.
Armageddon
You play as an avatar that is sent to earth from a divine realm to either destroy or defend earth. By the wave of your hands, you summon miracles that can then be used to defend, attack, destroy, revive the denizens around you. Lots of biblical references ahoy. Different characters will have different miracles they can perform. Attaining victory depends upon the map, but to summarize, you might need to destroy something, revive faith, perform miracles without exposing your divinity, etc. The game will be mostly through third/first person perspective. Your movement is handled via your hand motions, as is everything else you do. i.e. drawing diagrams in the air will summon certain miracles, while just leveling your hand in a direction will have you fly in a particular direction.
that's it for now, though there are probably a million things you can do with this.