Car Hero

This would be a device that went in place of a car stereo (and could also double as a car stereo). As the name implies, it would be a music game. The feedback would be mostly auditory and qualitative rather than score-based, and the game would work with the user's songs. So really, it'd be more like a cross between Wii Music and Lips (or Audiosurf/Harmonix's Phase) than Guitar Hero.

The auditory feedback can include both adding onto/"creating" the music (as in Wii Music) and sound effects that signify how well you are doing like fan applause/cheering.

For input, the driver's only option would be voice (singing, or possibly any sound effects that seem to fit the music), while the other passengers might have some sort of device which they can use to bang on like drums, or pretend to strum, or other things... whatever works. This would all be extremely drop-in, drop-out friendly, the players can switch "instruments" by just starting to do something different, and they can feel free to stop and start whenever, there's no failing ever. However, there might be "bonuses" (in the form of triggering certain feedback) for unison things, too.

Since the audience is pretty much captive, I don't think the game needs a lot of "game" (vs. toy) elements like scores and progression and such that typically hook and retain players, just immediate silly fun.

It would also be cool if passengers could plug in their mp3 players with USB cables and upload songs (or even choose from the ones already in the system!) to automatically form playlists (that is, if several players want to choose songs, the game would decide which songs and in what order to play and such)