Fantastic Four: 10 Theories For How They Can Join The MCU
4. Return From The Quantum Realm
Ant-Man and Ant-Man and The Wasp established the importance of the Quantum Realm, and the latter revealed that Janet Van Dyne had spent decades trapped there. Avengers: Endgame, meanwhile, used that mysterious dimension to help Earth's Mightiest Heroes travel through time, and it still feels like Marvel Studios has only really scratched the surface of what it means for this shared world.
In the comic books, the Fantastic Four frequently travel to a place called the Negative Zone, and tying that to the Quantum Realm wouldn't be remotely hard. If the team was established in the 1960s, it's feasible they've been trapped there ever since, possibly even the prisoners of Annihilus.
The actions of the Avengers in Endgame could have helped free the team, and there's definitely something to be said for them being fish out of water in the present day MCU. If nothing else, it would give the movie a comedic feel, and unlike Josh Trank's moody reboot, that tone does suit them best.
If nothing else, the team's origin story could at least be linked to the Quantum Realm in some manner.