As you know, Duvall was approached about being in III, but wanted too much money for Paramount to pay. So they put in Hamilton , and he was basically a cipher in the movie--much the best course of action for the Melanoma Poster Boy.

But, if Duvall had accepted, the script treatment would have had to be vastly different. In another thread you raised an early script treatment showing Tom skimming from the hotels. Probably there were others. The point is: Duvall would have been at least as important to the Trilogy in III as were Pacino, Keaton and Shire. I'm guessing that, given his declining role in the family at the end of II, the Hagen character would have been given some denouement in III--possibly a to-the-death struggle with Neri, and even a betrayal of Michael. Any way you conjure it, it'd be a big improvement over what we got in III.