I'd really like to see a "band" type of music game, with different instruments.
I mean, GH2 with 2 guitars is wicked, but imagine if there were a guitar, a bass, some drums, and maybe a microphone or a keyboard or something. Sure, it would be expensive as hell, but I'd buy it in a heartbeat.
Also, in the next GH, I'd like to see a couple fret buttons down by the strum bar, for those
really high notes
.