For me, I would have liked to see how Tom's relationship with Michael has progressed. At the end of Part II, they are quite cool to each other and Tom shows some signs of disgust, especially in his scene with Frankie Five Angels where he remembers how the Corleone family once was. All in all, I would have moved heaven and earth to get Duvall in the film. It would have been immeasurably better with him in it.