Originally Posted by The Infernal
Also, while Avengers may not have went with all of the founding members, there is an Ant-man film set for the phase two (which was probably why they didn't go with him). I'd be very surprised to not see him in a second Avengers film. I don't know about Wasp since I'm not aware if she's even in that film or not (though I doubt it would be a big problem to introduce her in A2).
Black Widow and Hawkeye definitely weren't founders, and yet they were included.
Marvel Studios never had any intention of remaining faithful to the actual order Avengers entered the roster. Or even of their actual status as Avenger (i.e., Hulk, the "Avenger" who was never anything more than a 2-issue honorary Avenger in the comics).