Beat Fever
Beat Fever is currently in the App Store. It's a really fun game. Definitely go check it out. I got to do some conceptual artwork for the game.
Through my concepts I proposed that as you're playing the game a massive monster would emerge from the shadows. Every time you hit an Incorrect Note, the monster would come closer and closer to the screen growing in power. If you hit too many Incorrect Notes, the monster would reach over the stage and eat your character + all of your bandmates. lol.
Hitting the Correct Notes would cause damage to the monster reducing it's health / pushing it away from the stage. Achieving a certain score would allow you to capture the monster.
You can use your captured monsters to do multiplayer battles with your friends. In a multiplayer battle you go up against 3 of your friend's strongest monsters.