Spoilers: (Highlight to reveal)
One of the great things about The Avengers is that the team is always changing. In fact Avengers #16 had Captain America replace the entire original Avengers team (of which he isn't even a member! He was introduced in Avengers #4!) with new members. The Avengers have always been mutable.

And so it shall be on film. At the end of Age of Ultron Cap will assemble a team that is a mix of familiar and new faces, leading forward into Phase Three. It's an exciting mission statement for Marvel Studios, letting audiences know that the Avengers team is more than just the guys who were in the first film. The make-up of that team is, technically, up in the air until that scene is shot (I suspect it's being saved for either scheduled reshoots or will be snuck into the schedule of Captain America 3, which goes before cameras early next year), but Joss Whedon's script did indicate who would be on that team. I won't reveal all the names - and some have changed in between drafts, by the way - but one character who has always been in this new team is Captain Marvel.