WHAT IF... Doctor Doom perfected his dimensional experiments?
After successful tests Doom perfects his plans, travels into the hell dimension and outwits Mephisto to free his mother's spirit (kind of like he did in Triumph and Torment but much earlier). Before ascending to a higher plane his mother begs him use his life for good.
Doom returns flushed with success and a changed man, and actually becomes friends with Reed Richards.
Later Reed invites Doom to accompany him on the experimental flight (moonshot, stardrive, whatever it is supposed to be these days). Doom, believing in Reed's capabilities, doesn't check the shielding and they are hit by cosmic rays.
All five return to earth with super-powers and form the Fantastic Five...
WHAT IF... Reed Richards and Doctor Doom both got their calculations right to begin with?
After successful tests Doom perfects his plans, travels into the hell dimension and outwits Mephisto to free his mother's spirit (kind of like he did in Triumph and Torment but much earlier).
Doom returns flushed with success and deciding to develop his new powers of magic, as well as finishing college with a reputation as a genius.
Later Reed puts adequate shields on the ship and the foursome return as heroes but without any super powers.
Don't know where this would go after that...
WHAT IF... Jesus not died? (in the Marvel Universe)
Jesus chooses not to die for the sins of man, but instead remains on earth to preach his beliefs, with diminished powers. Earth develops a different form of Christianity but still based on his beliefs and teachings.
As the centuries pass Jesus continues to use his abilities to help man, coming into conflict with beings such as Apocalypse, Selene, and Dracula, and living into the age of Marvels.
He becomes a founding member of the Avengers, even converting Thor to Christianity, and helping protect the world against the rise of a new generation of threats such as Dr. Doom, Magneto, and the Juggernaut, alien invasions, and supernatural incursions into the earthly plane.
Eventually, the star-god Galactus arrives to consume the earth. In the ensuing battle with earth's heroes the Silver Surfer is killed but Galactus cannot be stopped. Finally, rather than see his precious earth destroyed, Jesus chooses to become the new Herald of Galactus, pledging to lead him away from inhabited worlds and down a more peaceful path.
In return earth is spared, and once again Jesus becomes remembered for sacrificing himself for all mankind.