I'm pretty sure they've all contracted to do multiple movies. I think the plan is to first do sequels to the individual movies, ie Captain America 2, Thor 2 etc. The events in those movies will lead up to Avengers 2.

Yes: link

I've heard rumors that they were going to have a whole new cast. I don't know if that means they're going to have a movies with all the Avengers that weren't in the first one, or what. But, I haven't see anything concrete either way.

I know for sure that link, link and link all have their original cast. I really really doubt that after all of that they'd change the major cast members for Avengers 2. Also most of the actors have already signed contracts to do more movies.

link.
link and according to that article, Chris Hemsworth signed a similar deal.
Robert Downey Jr contracted for 4 films and his contract will be up after Iron Man 3 so it's possible they may recast him if they can't negotiate a new contract. However I think they will because otherwise they could have skipped Iron Man 3 and just saved Downey for Avengers 2.


It's recently been announce that there is going to be a S.H.I.E.L.D. tv series that will no doubt have an entirely different cast.