There has been a lot of speculations about the 2015 upcoming Avengers sequel. Here's my idea about the basic setting of the movie.
The old team members
One thing that people have speculated is that will the Avengers team keep fighting with same heroes along with the newcommers. This has especially been speculated with Iron Man, since in the end of Iron Man 3, Tony blew up all his Iron Man-armors and then had the sharpnel and the archreactor removed from his chest. Here are my theories on why the "big 4" members are not in the active duty on the hero team.
Captain America:
He is currently in medical treatment; the battle, in Captain America 2, with Winter Soldier and other possible enemy has left him so badly wounded that he is unable to battle, at least in most of the time in Avengers 2.
Thor:
The trailer for upcoming Thor: The Dark World shows, that Thor takes Jane Foster with him to the Asgard and also that Foster is (surprise) in danger at one scene, hold magically by movies bad guy. So there's two possibilitys; either Jane Foster dies and Thor pulls off Bruce Wayne-depression, thinking that Midgard(Earth) has no longer enything to offer him, so he doesn't want to come back. Other option, little more imaginative, is that Foster is send randomly into one of the nine realms, by the bad guy; so during the events of Avengers 2, Thor is desperately searching through the realms for his love.
Iron Man:
Pretty much how I explained it on above. Tony would still be there as kinda of hero and/or high tech consultant for Fury and a weapon manager for S.H.I.E.L.D.
Hulk:
Bruce Banner could be working for the S.H.I.E.L.D. with Tony. Fury might want to keep him as a "nuclear option" for crisis situations. My idea is that the movies villian(s) at beginning set a trap for Banner that wouldn't be in a way of their masterplan. Other possibility is that, if the rumors of Planet Hulk movie are true; Fury could've already launched Hulk into the where no green monster has gone before.
The new team
So besides Hawkeye and Black Widow, who would be the members of second Avngers team.
WarMachine:
Since he is already a government agent, it would be logical for Fury to try to recruit him to replace the Iron Man in the Avengers.
Falcon:
It has been confirmed that the Falcon will be portrait by Anthony Mackie in upcoming Captain America: The Winter Soldier. Now if the character is done justice in the film (he is, judging by the promo art), I really want to see him as a part of the Avengers.
Quicksilver and Scarlet Witch:
It was recently announced that
mutant twins will be joinig the team in Avengers 2. I like the characters and so I'm very Excited about seeing them on the big screen. My idea is that at the begin of the movie they are on the enemy's side, but then during the movie they would do reform and join the Avengers, kinda of the same way as in the comics; where they were first part of Brotherhood of Evil Mutants, before joining the Avengers.
Ant-Man:
Ant-Man's solo movie is due to come out a few months after the Avengers 2, but I would prefer if he made his debut already in Avengers 2. Since Ant-Man is not the most known hero to the big audience, it would be very good promotion for his own movie, if character first appeared alongside with Avengers. He is also one of those characters who can have his solo film just as well after his first debut. Will his partner, the Wasp, appear alongside with him; I don't know, really don't have opinion on that one.
Sum it up
This is my idea for Avengers 2-team. Joss Whedon hinted that he would be saving the Thanos for grand finale in Avengers 3, where all heroes from phase 1, phase 2 and possible phase 3 heroes, like Dr. Strange, would team up all together (maybe even with Guardians of the Galaxy) to battle Thanos' universal menace.