Isaiah Bradley
During the World War II, Black ops mission sent Bradly to stop a German scientist from recreating the Super Solider Serum. Upon his return, Bradley was captured for stealing the costume and shield of Captain America.
Bradley was imprisoned for two decades before he withered and died because of the flawed Super Soldier Serum which was injected in Bradley by US government during a test trial way before World War II.
Clint Barton
Popularly known as the Hawkeye /Ronin and just about touch and go title of Captain America.
Interesting, Tony Stark approached Barton to take over as Captain America since Barton was the only one who had the accuracy of throwing the shield. However, Barton declined as it felt disrespectful to his friend’s memory.