They can't use someone super famous to do the voice of Rocket. This aint Pixar. Unless the actor uses a voice other than his natural voice, but then whats the point?
A good (and cheap) option is to use a seasoned voice actor, someone who can come up with something truly unique. But the idea of this - "and Joe Pesci as the voice of Rocket Raccoon!" - just makes me cringe.
The voice will be just as critical as the CGI. Hopefully theyll try various things until its perfect. Stunt casting would be a mistake.