She-Hulk's origin is the easiest to bring in, since it's strongly tied to a character who already exists in this universe. If they go with the other obvious candidates (Ms. Marvel, Spider Woman, Wasp), they'd probably have to change their origins to be genetically engineered SHIELD agents. I can't see them introducing the High Evolutionary or the Kree (or even Hank Pym) just to bring in another Avenger.
Angelina Jolie would make a great She-Hulk, although she may be getting a little old for the role. But she could certainly bring the sass to the role.