SPOILER WARNING: The following article contains major spoilers for Marvel Legacy #1, on sale now.
The Marvel Legacy one-shot was jam-packed with reveals and surprises, but perhaps the most unexpected was the hint at a return for the Fantastic Four, who have been missing from the Marvel Universe since the end of Secret Wars. Unlike their fellow returning character Logan, the FF haven’t been dead for the last couple of years; rather, they've been off on a very important mission. It's pretty safe to bet, then, that anything that ends up bringing them back to the Prime Marvel Universe will no doubt be incredibly important and eventful.
A Whole New World
Prior to the FF's disappearance, the Marvel Multiverse had collapsed and been reformed into the singular Battleworld, with Doctor Doom as its God Emperor. Reed Richards survived the change to reality in his ark -- a hi-tech vehicle that protected Reed and a handful of heroes from the field of Doom's reality re-shaping influence -- but the rest of his family wasn't so lucky and were folded into the new history of Battleworld. There, Sue Storm married Victor Von Doom, while Franklin and Valeria were their children; Johnny Storm was turned into Battleworld’s sun; and The Thing was transformed into The Shield, a massive sentient wall which separated Battleworld from the dangerous southern territories filled with zombies, bugs and Ultrons.
Secret Wars ended — as it was always going to — with a clash between Mr. Fantastic and Doctor Doom, placed on an even playing field by The Molecule Man who had been managing Doom’s god-like power for him. When Mr. Fantastic forced Doom to finally concede that Reed would have done a better job with all that power, Molecule Man took that literally; he assumed Doom was conceding the fight, and that Reed should be the new wielder of the power of The Beyonders. Thus, Owen Reece gave Mr. Fantastic the ability to turn the world back into how it should be. Of course, there were a few changes here and there, such as Miles Morales living in the Prime Marvel Universe with a resurrected mother, a thank you from Molecule Man in return for previous kindness shown him by the young Spider-Man.
Following the restructuring of Earth-616, now known as The Prime Marvel Universe, the Future Foundation was tasked with rebuilding an entire Multiverse and had just the people to do it. The Molecule Man provided the raw power, anchored and channelled through the genius and precision of Mr. Fantastic; Franklin Richards — no stranger to creating worlds and universes from whole cloth — would dream up new aspects of the multiverse and Reed Richards would place them where they’re supposed to go. Then, an aspect of the Molecule Man would be sliced off and rooted in that universe to give it something to grow around while acting as a common bond throughout the myriad of alternate worlds. The job of the Future Foundation was to then explore and catalogue these new worlds as part of the new multiverse, in order to better understand the iteration of reality they had helped create.