The ability to play as real musicians, which prompted a number of lawsuits, will not be making a return in Guitar Hero 6.
If you relished another round of real-life musicians to play as in the next Guitar Hero game, you're going to be disappointed, as GH6 will not feature famous playable characters. That's not to say that real celebrity musicians won't be appearing in the game at all, but Activision has no plans to hand the reins over to the players.
Instead, the musicians will appear as NPCs in the game's quest mode, where players will recruit other rock stars and unlock song sets as they go. Metallica's Lars Ulrich and Gene Simmons of KISS are confirmed to be appearing, with Simmons narrating, and Ulrich seemingly one of the musicians you can recruit.
Queen, Black Sabbath, and, of course, KISS are confirmed as having songs in the game, and Guitar Hero 6 is supposedly a more "back to basics" affair than other recent titles in the series, with a "much bigger focus on rock 'n' roll."