What I read so far about Avengers:
Confirmed team members: Cap, Iron Man, Thor
Probable enemies: Loki, Hulk
Nick Fury will be in the movie. Other than that, the membership of the team is unknown. You can bet they'll have a female, and The Wasp is probably the most likely. I wouldn't be surprised to see Hank Pym to round out the original Avengers line-up.
I highly doubt Scarlet Witch... I can't see her powers translating well to live action. I expect they'll run with the original team.