Czar Alexander II of Russia and Frederick III of Germany. Both of these men were bent on reforming their respective countries. And both died prematurely, Alexander from an assassin and Frederick from cancer. If either or both of these men had lived normal life expectancies the terrible and bloody history of the 20th century might have been very different.
Agreed with Alexander II. You wonder if had he lived even just another 15 more years, Russia might have likely been that very different place by the early 20th century. He would have outlived his son who brought in all sorts of reactionary measures after he was murdered. And you also wonder if he and the things he was in the process of doing at the time of his assassination would have been more of that sort of influence on his grandson (Nicholas II).