So, a few years ago in Marvel, the Scarlet Witch (a mutant who can alter probabilities) went nutso and in the process, some of the D-list Avengers died. Later, she was found hiding in Genosha with Magneto (her dad). The X-Men and Avengers roll in because she's a time bomb waiting to go off. In a panic, the Witch re-writes all of reality on a scale she'd never shown the slightest capability of doing, creating an alternate history called "House of M" - a world where mutants run the show instead of humans. Eventually, the good guys remember the old timeline and the Witch undoes it all. But before the end of the series, she says "No More Mutants" and disappears. So, instead of the million plus muties in the Marvel Universe, only 200 or so were left (strangely enough, most of them were starring in Marvel Comics).
So I had the idea, what if instead she said "We're All Mutants" and instead of depowering a bajillion D-listers, every human on the planet gets mutant powers. Now, most of them are minor changes, like purple skin or the ability to move a penny with their mind. But like Magneto intended in X-Men 1, now everyone is a mutant.
Now, to make things easier, all of the non-mutant supers (like Spider-Man or the Hulk) got their genes rewritten so that they have the X-gene as the source of their powers. Folks like Tony Stark or Dr Strange either get cruddy little powers or evolve so they don't need their devices. (Iron Man as a ferrokinetic, for instance.)
And now there's a world full of super-powered people.
Thing is, I have no idea what the heroes of the game would be.