This requires you to unlock the following characters: The Hulk, Captain America, Thor, Hawkeye, Black Widow, Iron Man Mark 1, Iron Man Mark 6, Iron Man Mark 7, Iron Man Mark 42, Nick Fury, Agent Coulson and Maria Hill. All of these are unlocked via story missions except for the following:
Iron Man Mark 7 - You'll unlock this armor after the post-game mission "House Party Protocol," it costs 150k.
Agent Coulson - Coulson is unlocked via the side mission "Tabloid Tidy Up," which is located at the Daily Bugle (on the map, it's a red circle with a gold bugle in it).
Maria Hill - Hill gives you three missions on the Helicarrier deck (two races and a mission to unlock the Mini Helicarrier vehicle), after completing these she is unlocked.