He could decide that performing isn't what he wants to do. He finishes law school. Say Vincent takes over and with the advisement of Michael (who can still be alive since he died at a very old age), the Corleone family becomes the most power family in the states. There would be no major threats. Vincent, like Sonny, is very strong-willed, and dim-witted and creates a problem with a stronger family and he is killed. Michael comes to Anthony and convinces him that he can do what he couldn't do while he was don, and completely legitimize the family. I'd have a feeling that no matter what the storyline would be, any number 4 would probably end up being the worst of the movies, but its fun playing around with ideas.