Ms. Marvel
The character could be introduced in Captain America 2 as an agent of S.H.I.E.L.D. or maybe even Thor 2 in one of the many realms he visits.
She Hulk
She could be introduced in The Incredible Hulk TV show Marvel are thinking about next year as an ally to Hulk
Ant-Man
He could be breifly introduced as an agent of S.H.I.E.L.D. in Captain America 2(2014) before having his own movie in 2014
Wasp
She could be introduced in Ant-Man leading up to The Avengers
Vision
Could be intoduced in The Avengers 2 near the end when he turns to a good guy
Black Panther
Not really sure maybe briefly introduced in Captain America 2? or like Hawkeye maybe just appear in The Avengers
Red Hulk
He could be introduced in The Incredible Hulk TV show Marvel are thinking about making next year as a villain who eventually becomes a good guy
Namor
Another character that could really be introduced anywhere. Maybe Captain America 2 briefly? or even Iron Man 3 as a main character and ally to Iron Man or just briefly? or maybe The Incredible Hulk TV show Marvel are considering?
Quicksilver
Marvel would need to get the rights to The X-Men back to properly establish this character without making him not Magnetos son
Scarlet Witch
Marvel would need to get the rights to The X-Men back to properly establish this character without making her not Magnetos daughter
Falcon
Falcon could be introduced as an agent of S.H.I.E.L.D. in Captain America 2
Beast
If Marvel get the rights to the X-Men back then I would like to see Beast in an Avengers movie
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War Machine
The character War Machine was established in Iron Man 2(2010)
Iron Man
The Armoured Avenger was established in Iron Man(2008) and explored further in Iron Man 2(2010) and he was also in The Avengers(2012)
Captain America
Captain America was established in Captain America: The First Avenger(2011) and he was also in The Avengers(2012) where he was thawed out of the ice after being frozen in his first movie
Thor
The god of thunder was established in his standalone movie Thor(2011) and was also in The Avengers(2012)
Hulk
The Hulk was first established in Hulk(2003) and then explored once again in The Incredible Hulk(2008) but both of those films weren't a success unlike Hulks character in The Avengers(2012) which was AMAZING! Mark Rufallo has got Marvel talking about a new TV show next year and a film in 2015!
Hawkeye
Hawkeye was very briefly in Thor(2011) and had a really big role in the Avengers
Black Widow
Black Widow was established in Iron Man 2(2010) and her role was larger and her character was explored more in The Avengers(2012)