Anyone seen the harmonic table? Would be a pretty sweet layout for a game in my opinion.
Some sorta shmup could work, if you flew at the screen, the players position on the map could trigger the midi output, making the actual music relatively simple. Items/Power ups could add DSP effects/change the midi instrument. My friend had the idea that as long as you held a button, it recorded your position, so depending on the amount of buttons available you could loop that many tracks. That turns it more into a music making device than a regular game though. Not sure how to make it purely "fun" to play. Would need guns at the least. Maybe you could shoot your way through each beat, or it could kinda be like tetris from the blocks perspective, matched to music.