I realize everyone has different taste in music and the chance of any of these actually making in the game at some point are slim, but I'll recommend them anyways.
First and foremost there is a severe lacking of solid bass songs, don't get me wrong, there aren't any bad choices in the game, but there aren't really any songs that stand out on bass, most are overshadowed by guitar, drums and vocals. These tracks reflect a "lead" bass theme while still being fun on other instruments.
Tommy the Cat by Primus
One of the greatest bass players of all time, really any primus song would be good, but this song actually has a bass solo and the bass slapping parts look oh so much fun to play. Vocals would be interesting to say the least, and guitar and drums are still solid.
YYZ by Rush
Alas, because this song is strictly instrumental I don't think it will ever be considered, but really, this is one of the best instrumental songs ever written in my opinion. Numerous drum fills by Neil Pert, and a wicked guitar solo, this was the only song on guitar hero I would play the bass track for. Here's that wicked bass riff: http://ca.youtube.com/watch?v=iHPhyRyGAfc
Moving on to one of my favourite bands of all time, TOOL, and I can see TOOL has already been heavily requested around here. TOOL is in my opinion one of the most fluid bands of all time, all of their tracks flow so nicely and each instrument just compliments the other ones. The highlight of TOOL would no doubt be the drumming, with such odd timings I'd imagine these would be very difficult songs. Really any TOOL song would be good in Rock Band, but I think some of the better ones would be Schism, Lateralus, The Pot, and Vicarious.
I don't really think I have to say this, but I really hope they can snag at least one Pink Floyd track for this game.
Other notable songs I would like to see include Take the Power Back by Rage Against the Machine, Sound System by Operation Ivy, No Transitory by Alexisonfire, and Under Saturns Shadow by Tiger Army (not holding my breath for that one).
Thanks for reading, comments and criticism are welcome.