Michael Corleone has to be in the Fourth installment! Okay he died in GF 3 but that was years later when he was in his 80s so theoretically he can still be in it. I'd like to see just how good Vincent's regime is, I bet it will crumble and die and so Michael takes the reigns off him by "making an offer he can't refuse" so to speak and makes the business legit by passing the power onto Anthony who has become disillusioned with fame and treading the boards and finds that he wants to do his father proud.