Fantastic Four is in the main universe.

Ok so what if instead of being in an alternate universe, the Fantastic Four are trapped in an pocket dimension after getting their powers, one that gives them whatever they want similar to House of M. Sue wanted a normal family life hence the 60s dinner at 7 vibes. Reed wanted to create a scientific utopia thus the retro futurism. Johnny wanted to be a hero so they are the greatest heroes. And Ben wants acceptance which is why he's beloved by the people.

It would be a clever twist this far into the Multiverse Saga.