First of all, thanks to Malakin for mentioning Jeff Buckley. The 4-track tapes on his last "album" were a bit rough, but let's remember they were just 4-track tapes. With a bit of arrangement and studio sparkle, I'm sure Jeff would have turned them into masterpieces.
Led Zeppelin - I've not heard their last two albums (I only own IV at this point) so I don't know how they would have done had Bonham survived. I think Plant is still making great music even if he can't hit the high notes.
Jimi Hendrix - Wow, Hendrix is a tricky one. I agree with the poster above who said Hendrix might have experimented with funk. Listen to "Message To Love" and tell me that's not funk. I do know that Hendrix was getting weary of being a huge star, so I wonder what he would have done to avoid that stardom. Asking what kind of music Hendrix would have made after year 27 is like asking what God would have done with an 8th day. It blows your mind just to think about it.
I know he's not really "rock" but what about Roy Orbison? At the time of his death, he was still making great music and his voice was strong as ever. I don't know how long he'd continue had he lived, but I know he'd give us some great stuff as always.