Originally Posted by Chewy
Keep the team the same but add a new character that adds a new dynamic to the team.
Think Lando in ESB, Nightcrawler in X2, etc
More to the point: ESB successfully added Lando AND Yoda as two major heroes (as well as two prominent new villains, The Emperor and Boba Fett), and X2 added *three* new heroes (Nightcrawler, Iceman and Pyro).
If those sagas successfully added 2-3 major characters to the existing core group, it's safe to say that Avengers can do the same without alienating audiences.