What if James Dean hadn't died young?
I think he was great in all three movies he made, especially in East of Eden. In Giant he was good but not as great as in Eden, mostly because of the controlling nature of the director, George Stevens. But he became mythologized over his death, so he became an icon, even to this day.
Had he lived, IMO, he would have gone on to become a good actor, occasionally turning in great performances. But he would still be just another of the many good to great actors around, like DeNiro or Pacino. But he never would have risen to the greatness of Brando, who Dean idolized.