I would have wanted to see more of Vito and Michael interactions either at the very beginning of part II and then have Vito die then shown the flashbacks of his childhood and so on. I think that would have made an even better movie.

As of part 1, I would either have had Michael and Kay divorce earlier than in part II (Why drag that miserable marriage out?) or have Kay gotten married while Michael was in Sicily and Michael would have no choice but to move on. Then Michael would meet and marry a non whining woman.