1. Marvel could recast Iron Man

That's sort of obvious. And while Marvel likes constancy, they're no strangers to recasting characters.

We could think of Don Cheadle taking over James Rhodes from Terence Howard. Or, better yet, we could cite another case: the Hulk assigned to Mark Ruffalo after Edward Norton called it quits.

Another version of Iron Man would be more difficult to establish, since RDJ has left a deep mark on Marvel's character, playing him 5 times (without counting the brief post credit scene in The Incredible Hulk), whereas Terence Howard and Edward Norton played James Rhodes and Bruce Banner respectively only once.

However, that's one way to go. Marvel could, however, proceed another way.