The Doors, and what they may have gone on to accomplish had it not been for the death of Jim Morrison. Nirvana also comes to mind, with the passing of Kurt Cobain.
Bands where U say
I've yet to see a thread like this so here goes: what bands come to mind where you think what might have been? Perhaps they disbanded too soon, or a member or members died, leaving unrealized potential. I often wonder what would have come of the pretenders had honeyman-scott and farndon not bought the farm. I could list others, but would rather hear from you.