Great Men and Women That Could've Been

So, if you are not aware the Great Man theory is an idea that history should focus on a few individuals (such as Julius Caesar, Genghis Khan, and Napoleon) that managed to radically reshape the word in their lifetime. When I see the idea brought up here it always seems to be looking at what would've happened if one of these great men died relatively early in their career. I want to look at it the opposite way, can you think of anyone that had the chance to be a great man or woman if only they had had better luck.
 
how would I know such men or women? since they were not great history tends to forget them. All I have is a list of men/women who got great after their death for things that were really not that special in their lifetime.

Although on second look, I think Diego Riviera should today be considered not only as a contemporary of Picasso but as a painter just as revolutionary. He was! By all accounts! instead today everybody just knows him as that no-good husband of Frieda Kahlo.
 
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