7. Channing Tatum As U.S. Agent
John Walker first got his powers as Super-Patriot, an adversary/pro-wrestler that really stuck it to the anachronistic Americana image that Steve Rogers embodies. He provided such a stark contrast to that image that when Captain America stepped out of his wings and stripes, the United States Government asked Walker to take on the mantle he had so vehemently maligned. Walker took the opportunity and became the new Captain America, while Rogers took on the mantle of "The Captain", using the design of the US Agent suit. Once the Rogers took back his identity, Walker became US Agent, a somewhat gruffer version of Captain America who, unlike the original, is willing to work as an orders-are-orders agent of the United States Government. Having Channing Tatum and Chris Evans face off in this way would be a great plot for a third Captain America movie, especially following the potential fallout of any revelations laid in Winter Solider that may leave Captain America feeling less than thrilled with his country's government.
Tatum has the look, the size, and the action film chops to fill out a pair of armoured tights, as he proved in the much ridiculed, but not terrible GI Joe franchise. This casting would not only mean big dollars but would certainly be a great win for expanding the Captain America film world.