I wish someone would bring back the greatness of real-time music sharing from Turntable.fm, except maybe with instruments you could pick up and jam with. Maybe each person gets 1-2 loops, and 4-5 people get to be up on stage at once. Everyone else just gets little avatars that party.

Full real-time collaboration would be so annoying with various latencies. Would be super difficult to set up, but I've always wanted a way to do something like that too.