I'm going to go "out on a limb" and say Cullen would probably want more $$ than Marvel's willing to pay. They need to find someone who's willing to "branch" out and do a movie like this.
Seriously, Keith David is one those actors who could read the phone book and make it work. If his only line is "I am Groot", Hell, I could play Groot. Just help me get my SAG card and we're good.to.go.