9. US Agent
Let's get the important thing out of the way first, US Agent is the worst. He's a jingoistic jerk who was picked by the US government to temporarily replace Steve Rogers because of what a boot-licking yes man he was. He was basically the Captain America for the parts of the US population that keep their Make America Great Again caps next to their gun racks. That being said, he would make a very interesting contribution to the MCU.
The odds are good that Steve Rogers won't survive past Avengers 4Ever and even if he does, there's still the plot thread of him giving up his Captain America identity at the end of Civil War. The stage seems set then for either Bucky or Falcon to take up the mantle. Throwing in a character who perverts everything that Steve stood for would make an excellent foil to either of them.
It could also lead to a pretty damn interesting plot about who Captain America should be and what sort of values they should stand for in relation to the country and people that they represent.
Spoiler alert though, Captain America absolutely shouldn't be the guy who thinks Canada is just another state that got a bit uppity.