Captain America - Marc Blucas
Anyone who caught Marc Blucas as a semi-regular on Buffy: The Vampire Slayer will understand why I'd cast Mark Blucas in the part. He is ruggedly handsome without being too pretty, he is tall (used to be a basketball player, almost went pro, became an actor instead), well-built but in a gymnast-kind of build.
As an actor, he certainly puts the previous actors who've played Captain America (Matt Salinger, Reb Brown) to shame. And besides that, he's a decent actor - if you don't believe me, go track down some episodes of Buffy...
The Red Skull - Tommy Lee Jones
Tommy Lee Jones has certainly proven (at least to my satisfaction) that he can handle a role of Pure Evil, especially with his portrayals in...well, heck...just pick one. Fugitive. Men In Black. Small Soldiers. Rules of Engagement. He could certainly do justice to the role.
Secondly, just look at that face. Now, blank the hair so you have his skill. Now, apply a scarlet red tone. Sinister. Evil. The Red Skull just waiting to happen.
Namor - Timothy Dalton
Timothy Dalton has one of those faces which just screams "Aristocrat". Prince Namor is every bit as arrogant as any other royal scion, convinced that his birth places his above the rest. In addition, Namor has a very aggressive nature, and carries with him the aura of a predator.
Watch Dalton as James Bond in the Living Daylights, where he spends much of the film being angry at someone, and you'll see what I mean about his being able to carry off both the arrogance and the aggression of the Sub-Mariner.
Master Man - Arnold Schwarzenegger
Need I really say anything here? Master Man - the pinnacle of Nazi science, a powerful bulldozer of a villain whose sheer mass and power can destroy entire buildings. He's the
I mean, heck. Arnold even has the German accent, for crying out loud...
Sharon Carter - Julia Stiles
I was watching The Bourne Identity the other day, in which Julia plays Nikki, a minor supporting character who runs the logicistics of the Paris CIA cell for assassins. Watching her steal every scene she was in, I found it strange that a "name" star would do such a small role. I also walked out of the theater impressed enough to know who I wanted for Sharon Carter in a Captain America film!