i figured aside from Mckellen, Blanchet and maybe Weaving would return. The amount of characters from LotR's coming back is a little discomforting. I wouldn't want them to turn The Hobbit into a self reflecting circle-jerk. It should stand on it's own two (big) feet.
That being said, even if Legolas was in the books (which he is not), I would still protest Bloom coming back, becasue he is a terrible actor and it is a poorly written, boring character.