Thanks to Steamboat1874 for this idea. On the Cool Pictures of Music Icons thread, he said he would've really liked to see what Buddy Holly would've done with his music had he not died in that tragic plane crash. Got me thinking. So many musicians seemed primed to make a really great turn towards a new sound, but for being killed or dying before they got there.
Hendrix was experimenting with all sorts of sounds. John Lennon finally seemed to be at peace with his inner demons. Kurt Cobain was writing mellower, more introspective stuff. Bon Scott was at the top of his game.
As big of a Beatles honk as I am, the answer for me is SRV. That stuff he was putting out circa In Step after getting sober -- Tightrope, Life by the Drop, Riviera Paradise -- that was signaling a very cool new direction and I really would've loved to see what he created after that. Goddammit.