The theory solves a slew of problems for the Multiverse Saga that is scheduled to end with Secret Wars.

There are a lot of things that need to be set up if Avengers: Secret Wars is to be accurately depicted in the MCU. While the Fantastic Four will be making an appearance in their own film, it seems the main antagonist of their story is going to be Galactus. Doctor Doom, another character that is intimately linked to the Fantastic Four, needs to make an appearance in the MCU to function as a villain in Secret Wars.

A Reddit theory has emerged that neatly solves this problem from the MCU, looking to introduce Doom as a multiversal threat already, and making the character an integral part of the Marvel Cinematic Universe. This would also work towards bringing the Fantastic Four into the MCU, should they be part of a different universe altogether.

Doctor Doom could have been secretly defending the Earth 199999 from the Multiverse

The theory, proposed by user Cidwill, talks about how the Fantastic Four, should they hail from a different universe, could make their way into the MCU and integrate themselves with the superhero community and also set up their conflict with Doom.

According to the theory, Doctor Doom could hail from the mainline MCU, defending the universe from incursions, given that the character has been proficient in sorcery and science. The Fantastic Four could encounter him, and enlist his help to save their universe from an incursion.

However, when push comes to shove, Doctor Doom chooses to sacrifice the other universe to save Earth 19999. This would cause The Fantastic Four and a slew of assorted heroes from their timeline to come to the main MCU timeline, where they would work towards preventing Doom from fulfilling his multiversal aspirations.

The Fantastic Four could introduce alternate versions of characters that have already appeared in the MCU

If the Fantastic Four are indeed going to hail from a different universe, a slew of characters that are close to Marvel First Family could appear as a part of the universe that these characters inhabit. Black Panther, Namor the Submariner, The Skrulls, Impossible Man, The Inhumans, and Thundra all could make an appearance in the film, and perhaps could even migrate to the mainline MCU.

This would set up a lot of interesting dynamics (like the fact that there would be two Namors and two Black Panthers) along with introducing a slew of concepts (like the Inhumans) to the MCU, who have been largely absent since the failed show from the main Marvel Cinematic Universe.