First and foremost, let's just appreciate the fact that this movie will be an actuality one day. I, personally, have always loved the Fantastic Four as characters. They just never got the respect they deserved, movie wise. It's time for Marvel Studios to change their bad reputation, and make them relevant again. And here's the cast I would like to see to step up to the plate.
Michael Fassbender as Reed Richards/Mr. Fantastic
Bryce Dallas Howard as Sue Storm/Invisible Woman
Like Fassbender, Bryce Dallas Howard is also been in touch with a comic book movie in her past. She played Gwen Stacy in Spider-Man 3. And while that wasn't much of a big role at all, she's displayed her acting chops in movies like The Help and Jurassic World. Sue Storm is supposed to be the warm, motherly-like figure in the F4, and I could see Howard showing that character quality flawlessly.
Joe Keery as Johnny Storm/The Human Torch
Johnny Storm is always known as the young, cocky, but charming guy who gets girls like it's a pasttime. I think that role is right up Joe Keery's alley. He's not blonde like his comic-book counterpart, but neither was Chris Evans when he took on the role back in 2005. Putting hair color aside, Keery could balance the Human Torch in being that likable jerk. Oh yeah, and he's Steve from Stranger Things, so that looks real nice on his resume.
Brad Garrett as Ben Grimm/The Thing
He's got the voice. The height. And last but not least, the comedic personality. Obviously, he's much older than his other castmates here, but that's okay. I would envision he would only provide the voice and motion capture part for The Thing, so it's not like his face would be shown. One thing about Brad Garrett, is that's known for his voice acting and impressions. All he'd have to do is use his normal voice with a New York accent, and it's clobberin' time.
Jackie Chan as The Mole Man