Reed and Ben have been discussed pretty well here so far. Krasinski would be great but if his wife isn't Sue, then maybe make her Namora and let her attack the surface world looking for the long-lost Namor. That could give them great scenes together.

Might be controversial, but I liked Michael Chiklis as Ben.

Maybe Ben Whishaw as the Wizard. (He was Q in the latest Bond movies.) I'd like to see the Frightful Four-- and honestly, I'd rather NOT see Doom or the Surfer in the first movie. Give us Mole Man and set up Namor--that's plenty for a first movie to do.

Kathryn Winnick as the Invisible Woman. She is 43 and would be playing early 30s -- which she can do as she looks like she is in her early 30s. She is beautiful, likeable and can kick ass.

Thomas Brodie-Sangster as the Human Torch. He is 29 and would be playing 17 -- which he can do, as he has a tiny frame and looks somehow eternally preserved as a youth.

James Norton as the Thing. He is 35 and would be playing late 30s. He has the kind of rugged face and physique that conjures the spirit of Ben Grimm.

Possible villains:

Aforementioned Michael Shannon as Dr Doom.

Walter Goggins as the Super Skrull:

Toby Jones as the Mole Man:

Gabriel Ebert as the Impossible Man:

With some tweaking:

Concept poster with the proposed cast:

I love your choices for Reed and Sue, but Johnny - not so much.  I think Johnny's hair should be more along the lines of Sue's blond than that actor's dark dishwater blond.  And PLEASE do NOT suggest bleaching!!  

Also, there is just something not classically handsome enough about that actor to play Johnny.  When I think of Johnny, I think of a bit of a "golden boy" - almost a surfer type, but with a less casual edge.  More clean cut, etc.

Marc

For example:

Point taken thought that he might lack that all-American look. One of the problems that I think will always be lurking in casting Johnny Storm is that you'll get more of a man than a teenager -- or if you cast an actual teenager, you end up with someone quite different for the sequel. That was one of the problems with Chris Evans, who we know is a very good actor -- he just looked too filled out, too grown up (And he was only 24 when he appeared in Tim Story's film version). Brodie-Sangster sort of gets round that problem by being so wiry and child-faced. But I accept he might look a little more indie kid than golden boy.

The idea of Walton Goggins as the Super-Skrull boggles the mind, in a good way. 

A lot of better-known names were just too old or not suitable.

It makes you realise how hard these things would be to cast in reality -- here I can go through everyone and don't have to worry about availability, cost or if they would actually want to take the role. And even without worrying about those things, it's still tough!

I tried to think outside the box a little bit on this one, and I ended up with some people we'd have to steal away from the DC movies or who otherwise probably wouldn't take the roles, but here's what I came up--

Henry Cavill as Reed Richards (the hunkier Reed from the Kirby days)
Margot Robbie as Sue Storm (because--well, there's lots of reasons!)
Brad Pitt as Ben Grimm (pre-Thing Ben seemed pretty handsome and popular in comic flashbacks and losing that would add some real pathos to the movie version)

Not this particular kid as Johnny Storm (this is some model I don't know the name of) but he should look pretty close to this (and the right actor to play Johnny as a 15-17 year-old is probably 12 or 13 right now)

Christoph Waltz as Dr. Doom (I'm thinking an Austrian/German actor like him needs to play Doom and he was SO menacing in INGLOURIOUS BASTERDS!)

Danny Devito as the Mole Man (half of the characters he's already played have been pretty Mole Man-ish!)

Steve Buscemi as Puppet Master (those eyes! and he's the right age to play the father of--)

Michelle Williams as Alicia Masters (she rocks the short red hair Alicia should have and she has the acting chops to really play blind at the best level)

With this casting, I would watch a whole movie with just the Thing and Alicia too!