The original comicverse story of how Hawkeye joined the Avengers revolves around Black Widow (was a Soviet spy at the time. It was the 60s), and later, she joins up with them, too. But I agree that they should do a prequel movie with this origin story. It would be awesome.

Joss Whedon's lines and Robert Downey Jr.'s delivery are a match made in heaven.

Seriously. There should be more. Much more.

Did you not see Captain America? Because Steve is awesome and lovely, but he's kind of in angsty, angry "I lost my world and everyone in it" mode for Avengers.