There has been a lot of speculation as to what would be the "Mystery Movie" of Marvel's phase 3. We already know that some heroes will be getting solo movies while others may appear in ensemble movies following the success of the Avengers Franchise. If you look at what they seem to be building up to here in phase 2, It could very well be a Secret Avengers spin-off in phase 3 (or even beyond)
This depends on what may or may not happen in Captain America 2: The Winter Soldier. If you look at Cap's costume in this upcoming film - you notice it is none other than Steve Rogers: Super Soldier.
In the comics, it is the outfit Captain Steve Rogers wears after Bucky has assumed the mantle of Captain America. (I'm not even going to go into how Steve Rogers was killed off and then brought back because that storyline is just too incredulous for anybody at Marvel Studios to want to bring to the big screen.) So, for the sake of argument, let's just say after [Captain America: The Winter Soldier](movie:254973) plays out, Steve Rogers becomes disillusioned with all the political Bull Sh!t he finds himself in and no longer wants to be "Captain America" anymore.
He instead lets Bucky take over as Cap and decides to work as Commander Steve Rogers of S.H.I.E.L.D. Now, since we have already established in the Avengers' movie, that The World Security Council is not in love with the idea of a bunch of Superheroes being the last line of any defense, Nick Fury has the idea to make a covert group dubbed the Secret Avengers. *Spoiler alert*
At the end of Winter Soldier, Fury and Agent Hill are both hiding out underground and Cap is calling the shots. This could be the set-up to a covert group in Captain America 3 involving many of the characters mentioned below.
Fury enlists Steve Rogers, who then recruits Black Widow, Sharon Carter, Moonknight, War Machine, Hawkeye & Ant-Man (sorry, can't use Beast as he is Fox Studios.) The only character in that roster who has not yet been introduced is MoonKnight (the Ant-Man movie is slated as the last of phase 2) Let's just say that Steve Rogers knows of Marc Spector's Special Forces background and enlists him to the team even though the S.H.I.E.L.D. files on him list him as mentally unstable (He does believe he is the living Avatar of the Egyptian God of Vengeance, Khonshu) but hey, This team isn't worried about P.R. They have Covert Missions to go on.
Note: There would still be a "non-secret" Avengers Team that would have Bucky as Captain America.
As for the other members of that team? Iron Man (in his Black & Gold armor) Thor, The Falcon, Hulk, Black Panther, Scarlet Witch & Quicksilver.