While The Beatles and Green Day will take advantage of vocal harmonies, it seems Harmonix has other significant updates to make for the Rock Band platform. "As efficient as digital distribution is, there are still things that are much more efficiently delivered as a disc. In terms of major upgrades, I mean. That said, we haven't announce any formal plans for a sequel game ... yet."
Seems to me like Harmonix has something interesting to show us later this year. Let the speculation begin... now.