Been thinking what bands they haven't really had much songs for, or ones they never got. I could think of these:
Pink Floyd
Led Zeppelin
Black Sabbath
Rolling Stones
Muse
Smashing Pumpkins
Metallica
Plus tons of hits that are staples to rock music like Blinded by the Light etc.
Muse needs to never come to a potential new game for the sake of keeping a good joke alive.
All kidding aside, it was a shame that RB DLC ended when it did, since the GH contracts were in the process of expiring at the time. That's how we got Aerosmith just before the end, and I imagine they would have pounced on the Metallica stuff in an instant.