I think this is the right choice for Rock Band, but what is lost is the opportunity for instrumentals like "YYZ" or long jams like the end of "Free Bird".
Well, my idea is that some DLC songs could be categorized as spotlighting one instrument or another. Vocals, bass, guitar, and drums could still all be featured, but one instrument would be the star. Examples could include Led Zeppelin's "Moby Dick" for drums and Van Halen's "Eruption" for guitar. There could even be vocal spotlights like "The Great Gig in the Sky" by Pink Floyd.
In my opinion, this could add even more variety to this game.
If you wanted to take the idea even further, you could at a later time release expansion disks ala "Rock the '80s" which would spotlight one instrument more than the others. This way, you could have new takes on a sort of Guitar/Drum/Bass/Vocal Hero formula that would still involve the full band, but allow for more in depth experiences for each instrument.









