Live concert recordings often result in great "best of" collections of songs that would be great for DLC. Not only that, but many of these collection include special extended versions of songs that add to the concert feeling that can be so great in Rock Band. A couple of examples:
This has already been brought up several times in the Muse DLC thread, and with good reason. It is awesome for all the reasons I mentioned before: great setlist, extended song versions, and a concert setup that could be reflected in the Rock Band setting.
Pink Floyd: Pulse
This concert is incredible, even though some of the songs might not translate too well into Rock Band. Maybe some setlist editing would be required, but aside from that, the quality of the music and show make it a great candidate for DLC.
Incubus: Alive at Red Rocks
Another great example of an awesome setlist, from beginning to end. Plus an venue type that would be cool to see imitated in rock band.
Our Lady Peace: Live
Not only would we get a great setlist that spans most of their albums (even the best of the early stuff), they really ramp up the live versions of songs like The Birdman and Naveed in this concert.
Now if only they would release a DVD of Rage's performance at this year's Lollapalooza we would be set. Adding concerts like this would also allow for the addition of venues or gigs during career if purchased as an entire pack.
So post your favorite concerts recordings. All of these happen to be DVDs, but post live albums as well.