Harmonix has finally revealed their next project. It's called Chroma and it is a "music-driven FPS" where "music is your weapon."
Completely reimagining the classic Arena FPS formula, Chroma infuses fast-paced PC shooter action with music-driven gameplay. Join the fight as a Fader, a musical icon of the future, and battle for control of the all-encompassing cultural feed known as The Signal. In Chroma, music is your weapon. Sound is your arsenal.

They have even opened up a survey you can fill out to have a chance at joining the alpha for the game. You can find that right here while you check out some of the "key features" of the game, including the fact that it will be free-to-play upon release.
Music-driven FPS gameplay Music is your weapon Band together Conquer shifting terrain