What I think would be great is if they would put together some bands from novelty bands like Dethklok or Tenacious D or Flight of the Conchords.
Anything by Dethklok would be hard as hell but fun. May I suggest "Go Forth and Die" or "Murmaider."
Some older Tenacious D like "Tribute" or "Wonderboy" would rock.
Flight of the Conchords might be harder to pull off. "Bowie," "Rock the Party," or "Robots" might work. Though they may not all be fun on all the instruments.
Also, some Primus would be great. "John the Fisherman" was one of the best songs on Guitar Hero 2, and if you add vocals and drums, it would be perfect.
Finally, where's Alice Cooper and Queen?
And more Rush!