If the Fantastic Four are rebooted for the Marvel Cinematic Universe, we can look forward to a whole new cast for Marvel's First Family. Of course, such a possibility is still some way off, as it will take 12 to 18 months for Disney's acquisition of 21st Century Fox to go through (and that's if it doesn't get blocked by the Department of Justice), but it's not too early to start looking at the current rising talent stars who could take on the roles of Mister Fantastic, the Invisible Woman, the Human Torch, and the Thing.
Twentieth Century Fox has so far made three Fantastic Four movies: Tim Story's Fantastic Four and Fantastic Four: Rise of the Silver Surfer, and Josh Trank's somewhat notorious 2015 reboot, which the director has since thoroughly disowned. None of the previous movies were particularly well-received by critics or fans, so if the Fantastic Four join the MCU, the door is still open to create the definitive version of this particular superhero team.
Ideally we're looking for actors who have proven themselves, but aren't yet leading their own franchises (because being an MCU superhero is a big time commitment). There's a lot of talent out there, but these are four actors who could be great as the founding members of the Fantastic Four.
This Page: Mister Fantastic and the Invisible Woman
Matt Bomer as Reed Richards/Mister Fantastic
Although he looks like he stepped straight out of a comic book, Matt Bomer has never played a live-action superhero - though he did voice Superman in the 2013 animated film Superman: Unbound. Bomer has classic action hero looks, but also a great deal of range: he played a male stripper in the Magic Mike movies, a lethal hit-man in Shane Black's crime comedy The Nice Guys, and a glamorous vampire (glampire?) in American Horror Story: Hotel. He's also the perfect fit for a romance storyline between Reed Richards and Sue Storm; one of his first major roles was as the first season love interest, Luc, in the Eliza Dushku-led sci-fi series Tru Calling. We've already seen Ioan Gruffud and Miles Teller in the role, but Matt Bomer has what it takes to become the Mister Fantastic.
UPDATE: John Krasinski has thrown his hat in the ring, saying he'd like to play Mr. Fantastic.
Johanna Braddy as Sue Storm/Invisible Woman
Johanna Braddy is currently best known for her TV roles in shows like Video Game High School and UnREAL, and has a talent for playing sweet, earnest and strong-willed characters. Aside from looking uncannily similar to the comic book version of Sue Storm, Braddy is an excellent actor with experience in action roles - in particular, as FBI Agent Shelby Wyatt in ABC's Quantico. In UnREAL, she played an accomplished attorney who is cast in a reality TV dating show, and has to try to hold on to her humanity in an environment where producers are trying to push her into ratings-grabbing cat fights and back-stabbing. Sue Storm is a challenging role, since whoever is cast will have to balance the turmoil of being superpowered, a romantic storyline with Reed Richards, and strong family dynamics with Johnny Storm. Jessica Alba and Kate Mara played Sue in the previous movies, but this could be a perfect breakout role for Braddy.