It seems to matter little enough that the president of Marvel Studios himself said that it will take us a long time to see the Fantastic 4 debuting at the MCU because the fans speculate with whatever clue what is needed.
The last theory of the community has been that David Harbor, the actor of Stranger Things and Hellboy, was going to play La Cosa, also known as Ben Grimm, member of the Fantastic Four, in Black Widow.
All this mess is given because Harbor defined his role in Black Widow as "a thing" or "a thing" in which he was involved. So some viewers with a lot of imagination came to the wrong conclusion. The actor took advantage of his passage through Wired to deny all this.
The actor clarifies that it is an expression that he uses continuously and that is not any kind of clue about his role in the new Marvel movie, a role he has not revealed, of course.