Brad Pitt as Thor - Tom Cruise as Iron Man - Eddie Murphy as either War Machine or the Falcon - I can see the first two but not the third. I think Eddie Murphy would have wanted (and Marvel would have wanted him) to play a villain. An actor can be more over the top as a villain.
Will Smith might have made a good War Machine or Falcon. But even he, I think, would make a better villain. I think I generally just want comedians to be villains.
Ralph Fiennes might have made a good Bruce Banner, although the shape of his head is not as well suited to the CGI Hulk as Mark Ruffalo's head shape is. Same comment for Denzel Washington. Mark Ruffalo just has that perfectly shaped head.
The thought of Tom Cruise as Iron Man makes me sick.