For years, some vocal fans have clamored for John Krasinski to play Reed Richards, the head of the Fantastic Four. There have been two major attempts at adapting the story of the Fantastic Four before: the 2005 and 2007 movies, which starred Ioan Gruffudd, Jessica Alba, Michael Chiklis, and a pre-Captain America Chris Evans, and the 2015 film, which starred Miles Teller, Kate Mara, Michael B. Jordan, and Jamie Bell. When Disney acquired Fox, it opened up the door for the Fantastic Four to come into the MCU. But it seems that, unlike the X-Men, Disney isn't interesting in bringing back those actors.
Marvel has a "Fantastic Four" film in development, and it's possible that Krasinski will play Richards in that film. But it's also possible that his casting in "Doctor Strange and the Multiverse of Madness" was just a fun Easter egg for fans, and they'll ultimately go in another direction with the character.