Eleanor Roosevelt? Maybe a reaction to a truly nutty Republican ticket (McCarthy/LeMay) Eleanor sneaks in as a VP choice (maybe under Stevenson) and then POTUS dies in office? Long Shot...
MLK? Again, a long shot. He doesn't die in Memphis, Nixon fights tooth and nail, barely survives impeachment, but ruins the Republican brand. Carter wins 1976, with MLK as VP. Carter dies in office ...
Colin Powell - obvious - Clinton scandel prior to 1996, Republicans run Powell against Gore in 1996.
True outside-the-box:
Amelia Earhart. Completes her trip. Is at Pearl on Dec 7 - gets into the air in a P-40 - somehow. Shoots down some Zeros and Vals - becomes an even bigger heroine. No more combat in WWII, becomes General of WAAF, shuttles bombers across the Atlantic. Leading advocate of women's rights. Comes out early against Vietnam, becomes darling soldier-woman of the left. Goldwater/Lemay beats Nixon but then blows up even more spectacularly, and McGovern/Earhart wins in 1968. McGovern dies ...
Mike Turcotte