Bradley Cooper is in negotiations to join the cast of Guardians of the Galaxy as the voice of Rocket Raccoon, and Elizabeth Olsen to play the Scarlet Witch in The Avengers: The Age of Ultron, according to Hollywood Reporter.
 
The Hangover star is well known for his impressions and distinctive voice, and it would make for some interesting verbal parrying to see him opposite Vin Diesel as Groot (see "Vin Diesel in 'Guardians'").  The gun-toting, wise-cracking anthropomorphic Rocket is a fan favorite, and the writer/director James Gunn has said several times that Rocket is the "heart" of the team.
 
Elizabeth Olsen (Oldboy) is in talks to play the Scarlet Witch in The Avengers: The Age of Ultron.  There have been rumors as to who will be cast as Quicksilver (see "Possible Quicksilver for 'Avengers 2'"), but no confirmation.