I REALLY hope Whedon returns for the sequel. Nothing sounds better to me than the words "Joss Whedon's Avengers trilogy".
How important do you consider getting Whedon back for the sequel to be? Do you think Whedon was an important reason for how huge Avengers turned out or do you consider him a director for hire?
I think that he needs to write or co-write the sequel in order to keep things consistent, including the quality of the scripts - he's possibly the best there is at what he does best. A change of director might benefit the franchise, though. Would love to see Guillermo del Toro do it, but his work on other projects (particularly the Hulk TV show) might preclude any involvement. Drew Goddard, maybe?