Rock Band needs something absolutely ridiculous for guitar. If it weren't for the mic instrument I would suggest something like one of Speed Metal Symphony - Cacophony, Mable's Fatal Fable - Jason Becker, Far Beyond the Sun - Yngwie, or Get out of my Yard - Paul Gilbert.
But since it kind of needs, or should have lyrics these wont work, but this still leaves some great options available that are still absolutely insane. A few Cacophony songs would still be awesome, such as Savage, Where my Fortune Lies, Stranger, or Sword of the Warrior (among others that would be good). Pretty much anything by Impellitteri would work. Even Sea of Lies by Symphony X would be good.
But the way I see it; if you have some far more intense pieces for guitar you will probably attract all of the Guitar Hero veterans who aren't already sold on Rock Band. Plus the people interested in that kind of music, but I would assume it would sell the game on more GH players than people who like shred
You guys did excellently with GH2 by putting Jordan in. That was a great addition and was excellent for people interested in a huge challenge. My suggestion is to stick with that, put something extremely difficult to do well in addition to the rest of the more relaxed tracks.