With the rights to the Fantastic Four now back at Disney, it's already been confirmed by Marvel Studios president Kevin Feige that fans will get to see the introduction of Mister Fantastic, the Invisible Woman, the Thing and the Human Torch in the Marvel Cinematic Universe. For the time being, there is no official release date for a Fantastic Four movie, and there are no clues as to how Feige plans on introducing Marvel's First Family.
For the time being, all fans have are guesses and theories. Of course, there is always the possibility that Marvel will introduce the Fantastic Four in a linear capacity, perhaps to try and fill the void left by the original Avengers after the events of Avengers: Endgame. However, a fan theory on Reddit suggests a Fantastic Four film should take place in the 1960s -- and frankly, that is a great idea.
The very first issue of Fantastic Four by Stan Lee and Jack Kirby was released in 1961. Therefore, the superhero family's first adventures were set firmly in that era, and the crazy science they encountered and wild adventures they went on are essentially a reflection of that time. For that reason alone, a film set in the '60s would not only honor the comic book's origin, it would also allow the Fantastic Four's first adventure in the MCU to be as faithful to the source material as possible.
Having the film set in this specific decade would bring its own aesthetic which would immediately set it apart from other MCU entries. The era would also allow Reed Richards' retro-science to be on full display, and that on its own could make the movie stand out even more. Furthermore, having the film set in the '60s would also open the doors for storyline possibilities. In fact, since S.H.I.E.L.D. was originally founded in the 1950s, the Fantastic Four film could show us how the secret spy organization fared in its first few years in operation. Marvel is no stranger to having films set in the past, and these have contributed to further fleshing out the history of the MCU -- S.H.I.E.L.D. included.
Captain Marvel showed fans the organization in the 1990s, and how Agent Fury first came into contact with aliens. Even the Ant-Man films showed us flashbacks to Hank Pym's time at S.H.I.E.L.D., and how he worked for them as a superhero. Therefore, a Fantastic Four film set in the '60s could show fans how S.H.I.E.L.D. began to operate with retro-gadgets and pseudoscience -- the likes Reed Richards specializes in.
Now, if the introduction of the Fantastic Four takes place in the '60s, then the big mystery would become how the team could be active in the presence of the MCU for team-ups with other superheroes. Well, the answer could be as simple as what happened with Steve Rogers and Carol Danvers -- one was trapped in the ice, while the other was on the other side of the universe.
The nature of the Fantastic Four's adventures takes them in some strange and unexplored new locales such as the Negative Zone, a parallel dimension. If Reed, Sue, Johnny, and Ben all end up in an alternate reality, they could be stuck there for years. They could even experience time differently, which would explain why they haven't aged a day between the '60s and the present.