A lot of cast Rhodey has to do with who is cast as Tony. I think they need to be around the same age. That said, I can see a handful of candidates.
Michael Jai White: Look wise, he's Rhodey ripped straight from the comics. But with all due respect, he's not the greatest actor, just a passable one.
Henry Simmons: Though I'd much rather see him get Luke Cage, he could definitely play Rhodey. Great presence, commanding voice.
Shemar Moore: A little lighter than Jim is usually colored, but other than that, has the look. He's a solid actor. Rhodey has at times been written as a bit of a player, and Shemar could definitely highlight that part of the character. I think Jim as a character is 'harder' than most of his roles, I'm not 100% sure if he can deliver that part of the character.
Mehki Phifer: I think he's a very good actor, woefully underrated. I think he can definitely play the role well.
Derek Luke or Omar Epps might work as well.
Jamie's probably not an option. If he was too big to take the lead in Luke Cage, why would he want a supporting part in Iron Man?
Tyrese? Christ, no. I don't want him near any superhero. I'm hoping and praying Cage falls through on him.
Ice Cube, I like, but not for this particular role.