Being a vocalist as one my primary instruments I would love to see some show music in the game. Not a whole lot, as many times the other instruments would be left in the cold (or boring) but there are some really great vocal pieces out there.
Ones off the top of my head that could work in the game:
Rum Tum Tugger from Cats
Skid Row/Downtown from Little Shop of Horrors
If I Were a rich Man from Fiddler on the roof (there are some really intricate string time stuff, and horns and other stuff to keys and a solid bass)
One Night in Bangkok from Chess (as this one had radio play it's even more Rock Band Friendly).
Wouldn't want it to dominate the choices - but a few choices picks for vocalists (especially for harmonies/parts) would be fantastic.