As we all know, Phase 1 was about introducing the characters, establishing that they shared the same universe and then bringing them together in 'The Avengers'.
It would be understandable to assume that, following on from 'The Avengers', Phase 2 would be leading to 'The Avengers 2'. However, I think the storyline will split in two and Phase 2 is in fact building to 'The Avengers 2' and 'The Avengers 3'.
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Storyline 1 continues the Marvel Cinematic Universe as we already know it and will consist of the following films;
The Avengers /
Iron Man 3 /
Captain America: The Winter Soldier /
The Avengers: Age of Ultron /
Ant-Man
This storyline is based on Earth, led by Science related events, will feature S.H.I.E.L.D and ultimately lead to the Ultron threat in 'The Avengers: Age of Ultron'
The main characters are Tony Stark, Captain America, Black Widow, Nick Fury and S.H.I.E.L.D.
Although it is released after 'The Avengers: Age of Ultron', I think the Ant-Man film may be part of this storyline, especially if the 1960/70's setting is retained;
http://www.comicbookmovie.com/fansites/SuperheroSpeculation/news/?a=80908
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I will call Storyline 2 the Marvel Cosmic Universe, and it will consist of the following films;
The Avengers /
Thor: The Dark World /
Guardians of the Galaxy /
Dr Strange /
(Thor 3?)
(As yet unknown films; Ms Marvel, Nova etc?)
The Avengers 3
This storyline will be set in space and/or on different worlds, expanding the cosmic side of the Universe. It will introduce lots of new concepts, technologies and species and ultimately lead to 'The Avengers 3'. I assume Avengers 3 will have Thanos as the main villain, and perhaps be a version of The Infinity Gauntlet comic.
The main characters will be Thor, Dr. Strange, Guardians of the Galaxy, with spin off aliens likely to be introduced.
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So, in summary;
Phase 1 - introduce new characters, show shared universe, lead to Avengers
Phase 2 – Finish Iron Man's story arc, continue Thor and Cap's arc, introduce new characters, lead to Avengers 2 and 3.
Phase 3 – Finish Thor and Cap's arc, introduce new characters, lead to Avengers 3.
(If I am correct then this trend could continue with perhaps some Phase 3 films, such as Cap3, even be building towards Avengers 4...)