First of all, Rock Band: Beatles. Don't let the guitar hero folks steal this from ya. I want this, everyone wants this-- those three little words just scream "please give me millions of dollars". You're the innovators, you deserve this.
And it goes without saying to anyone who read this journal on a regular basis how much I'd shit myself for Rock Band: Talking Heads.
Second of all, fix the drum samples that are used before the song and during solos. You guys get access to the master recordings, right? A perfect .wav file with all the right effects already applied to it, you don't have to isolate anything-- just take a sample of the kick drum, snare and whatnot from that song and use those sounds instead of the painfully MIDI ones that never sound natural when they suddenly hit instead of the expected recorded sounds.
Third, I'd like a "tone deaf" option for singers and an equivalent "I suck but want to play" option for all instruments. Sometimes, esp. when first learning how to play or very drunk, you just want to do your damnedest and not worry about how close you really are-- sometimes you don't want a challenge and after all, games are made to be
fun. This would be pointless in 'game mode' but awesome for quickplay.
Last, duets or at least multiple mics for backup vocals.
But mostly-- thanks for giving me an excuse to sing loudly in my living room.
If you think all this is overhype:
Watch that and tell me this isn't going to make music better over the next 20 years. Learning the basics of rhythm and how to play an instrument is now a game. GO KID, GO!