Re: Downey's last?
That's a tough one I mean if their looking for an actor in their early 40's by the time film starts rolling than that actor would have to be in his 30's at this time.
I'd go with Colin Farrel since he's a former Hollywood badboy as RDJ was.
A few of the Marvel Superheroes are somewhat niche characters that aren't extremely versatile, thus they don't have a lot of longevity potential. For example, Namor is a water guy, Silver Surfer is a space guy, Dr. Strange is a magic guy, and so on.