Originally Posted by Tony Stark
To me Tony, Steve, Thor and Bruce all have to be there. They are the Kirk, Spock, Bones and Scotty of this series.
In the comics, the Avengers featured "The Big Three": Thor, Cap and Iron Man. Hulk was never officially a member and left the group after just 1.5 issues, although he always retained an honorary Avenger status.
In the MCU, it seems pretty clear that they've bumped that up to "The Big Four," so I still expect to see Hulk as part of the team for many episodes to come.