Says the Deadline story: "Not sure the character, but evidently it’s a love interest for Steve Rogers (Chris Evans). I’m told that Game of Thrones' Emelia Clarke, Downton Abbey's Jessica Brown Findlay, Warm Bodies' Teresa Palmer, Knight Of Cups' Imogen Poots and The Five Year Engagement's Alison Brie are testing for it."
Alison Brie, of course, is better known to fans as Annie Edison on NBC's Community, a show on which the Russos have worked pretty extensively.
The role in question is likely to be that of Sharon Carter, Captain America's girlfriend in the 21st Century and the niece of his girlfriend back during World War II, who appeared in the first film and was referenced in a deleted scene from The Avengers. Carter played a key role throughout the Winter Soldier story that the film is adapting--one which only grew as the next few stories under Ed Brubaker happened.
The report also suggests that Scarlett Johansson's Black Widow might "stop by," a rumor that makes sense given the comics it's adapting. Black Widow's relationship to the Winter Soldier program was a staple of many of those stories, since both of them operated in Russia/The Soviet Union.