The team of superheroes wouldn’t have to show up in the middle of the movie, in fact, it would probably serve the Fantastic Four and the MCU as a whole best to have the Mister Fantastic, the Invisible Woman, the Human Torch, and the Thing pop up in one of the signature post-credit scenes to get fans excited for the next phase.
It’s a matter of when, and not if, for these fan favorites to show up in the MCU.
If Marvel were to bring Doctor Doom in for Black Panther 2 then it would be the perfect opportunity to usher in the era of the Fantastic Four and give the characters a decent film adaptation for a change.