The most tragic of X-Men stories will never be adapted into the MCU, for in this tale, Professor X becomes the mighty Juggernaut and dooms the entire Earth - and himself. The Juggernaut is one of the X-Men's greatest foes - even though Cain Marko is notably not a mutant - and has menaced the group for decades. But in What If...Professor X Had Become The Juggernaut?, written by Kurt Busiek with art by Vince Mielcarek and colors by Tom Vincent, Professor Charles Xavier walks down the wrong path in life and pays dearly for his mistake.
In the main 616 universe, Charles Xavier follows his stepbrother Cain Marko into a cave during the Korean War (in which Cain planned to desert and hide out during the fighting). Cain touched the Ruby of Cyttorak - which turned him into the unstoppable Juggernaut, complete with super-strength, durability and the ability to survive without food, water or even air. But in this particular What If story, it was Professor X who touched the Ruby, it was Cain who escaped the subsequent cave-in, and thus Xavier was buried under hundreds of tons of rock for years.
Since Xavier didn't found the X-Men when Magneto broke into the missile base (during the X-Men's first adventure), he was repelled by the Fantastic Four. But with no "good" mutants attacking Magneto's brotherhood of "evil" mutants, anti-mutant hysteria raged across America. When Xavier finally freed himself after years, his optimism about mutant-human relations had turned to anger. With his newfound powers, he was able to quickly end all conflicts between human and mutant; he never harmed any mutant, but any human who went against his wishes was severely punished - and those humans who attacked mutants were met with a swift end.
With the help of the Juggernaut, mutants quickly conquered the world. Humans were subjugated and quickly became second-class citizens. Cyclops eventually decided enough was enough and formed a band of mutants who desired peace among the two groups, instead of a system of one above the other. With Magneto's help, the rebels staged a trap for Xavier about Asteroid M that involved hurling the Juggernaut helplessly out into space; with his powers, he will float aimlessly for eternity. "Nothing can stop the Juggernaut," muses the Watcher's narration. "And nothing every will."
Unfortunately, while the film rights to the X-Men are back in Marvel's hands, it's highly unlikely a What If? story will be made before both Xavier and the Juggernaut make their proper appearance - and even then, both characters have had two incarnations already. It's more than likely that Marvel will stick with new characters for the foreseeable future. This means that the tale of Professor X's rage in the body of the Juggernaut must remain on the page for now.