C'mon, a guitar or flute or something to make a sweet song with my friends and maybe a way to record the songs and play them back.
As a musician, I'd love to see musical instruments in the game. You could maybe have a sheet music styled interface, so you can pre-program the instrument to play a little tune.
Think of Lotro/Mabinogi style of instrument playing. The instruments would just play a song based on a text document you have somewhere in the game files. and players would sync up performances with some sort of console command.