Anything by Dream Theater. I'd love a Dream Theater specific game actually. Songs like Metropolis, Constant Motion, Ministry of Lost Souls, the Budokan version of Hollow Years, Under a Glass Moon, Overture 1928, Lie... I could go on and on. I want Guitar Hero: Dream Theater. Now.
If we're getting more Hendrix I'd love All Along the Watchtower and Voodoo Chile too.
One of RB2's 20 free downloadable songs is IV by X-Japan. It seems fun, I've only played it once.
I'm sure I'm the minority, but I'd STILL love to see Kutless or other Christian rock. Yeah, I said it. I know the Kotaku boards would be in a frenzy (Spirit in the Sky STILL gets people crying over "a Christian song in a rock game", and that Country pack... how DARE they )
Maybe more 90's rock. STP still needs more of their songs put out (I'm still waiting for Wicked Garden).
Not EVERY song that is released for RB should be a chore to play. The best example of difficulty and fun was "Hysteria" by Muse, as it's probably the prototype for songs in a game like Rock Band. When they had that 3 month stretch of nothing but generic metal that was basically impossible to play, I was turned off of DLC until they started weening off metal and gave us something worth playing.
Plus... I'd LOVE to see a Beets pack. Couldn't be any worse than the OTHER Nickelodeon pack they released a few months ago...
Plus, more country! I feel like such a redneck playing Miranda Lambert, but dammit, it's a lot of fun.