I absolutely hate Wasp. The cartoon ruined her for me. She Hulk is cool but isn't she really just Hulk with boobs and a vagina? Its sort of a been there done that thing.
She-Hulk is unique for her personality, though. She's an amazingly fun character, but one I'm afraid only works well in comic. The best bet for a new team member is Wasp, since she can very easily be introduced in Ant-Man. Outside of that, and unless they have planned to give a superheroine a solo movie (and I really doubt it), I could only see Ms. Marvel getting some time in Captain America 2.
I asked this in the Iron Man thread, but why didnt Shield recruit War Machine over Stark? Stark was ineligible because he was all self centered and stuff, but War Machine is a disciplined soldier witht he same (or at least similar) suit of armor, so why not choose him? Or both?
Cap was chosen because he had experience with the Cosmic Cube. Thor was there because he's Loki's brother. Hulk was there because Banner is an expert in gamma radiation. And Iron Man is there because he's an expert in renewable energy sources. You have to understand that the Avengers themselves aren't created till the later half of the movie, and they had no intention to do so until The Son of Coul died.
By the time they form the Avengers, they simply go stop Loki. No time to call War Machine. I'm sure now that the team does exist, they'll give him a call for Avengers 2. Especially since he is showing up in Iron Man 3.
Well after Thanos, the third Avenger movie could feature the Masters of Evil for enemies. You'd have Red Skull and Blonsky team up with what ever other bad guys show up in the movies.
Masters of Evil would work well as a second movie, they'd really be underwhelming after Thanos. Building up the big Cosmic bad guy for some more, while giving the heroes a team of supervillains to face (something we really haven't seen on film either, outside of the Brotherhood of Mutants) seems more suited for the second movie.
You have Abomination and the Leader, Red Skull is still missing, and I'd say we have good shots at both Enchantress and Baron Zemo in Thor 2 and Captain America 2, not to mention Mandarin and whoever the villains are for the other two movies before Avengers 2 (one of which is probably Ultron for Ant-Man).