According to movie scooper Daniel Richtman (per ), Smith was offered the role of Reed Richards, aka Mister Fantastic, shortly before the SAG-AFTRA strike began.

Source: https://www.cbr.com/rumor-matt-smith-fantastic-four-reed-richards/

Doom's rivalry with Reed Richards has defined his adult life.

Source: https://www.cbr.com/marvel-villains-got-revenge-they-craved/

Immediately, the Thunderbolts are mobilized and begin apprehending the likes of Susan Storm, Reed Richards, and dozens more.

Source: https://screenrant.com/spiderman-stronger-hulk-marvel-comics-confirms/

Meanwhile, John Krasinski (Jim Halpert) has become an action hero, featuring as Reed Richards in Doctor Strange in the Multiverse of Madness.

Source: https://screenrant.com/the-office-reboot-dependent-one-person-nbc-response/

Now, speculation points to Mila Kunis potentially being someone Marvel is eying to play Sue Storm aka the Invisible Woman, sister to Johnny Storm's Human Torch and eventual wife of Reed Richards' Mister Fantastic.

Source: https://screenrant.com/fantastic-four-movie-cast-sue-storm-mila-kunis/

Reed Richards from the Ultimate Universe started out a lot like his Earth 616 counterpart.

Source: https://www.cbr.com/ultimate-marvel-the-maker-secret-wars/

The latest Sue Storm casting rumors arrive shortly after reports of Adam Driver being close to closing a deal to play Reed Richards, leader of the Fantastic Four and husband of Sue Storm.

Source: https://gamerant.com/fantastic-four-mcu-cast-vanessa-kirby-jodie-comer-sue-storm/