My tuppence worth...

I would be curious to read any treatments of a fourth movie that Puzo may have been working on prior to his death, but I don't believe that a fourth movie should be made purely by virtue of the fact that Puzo would not be an active part of it. Although FFC should be given rightful credit for the way he steered the character of Michael Corleone, the lack of Puzo's unique understanding and interpretation of omerta would cause the film to lose a huge factor of what make its predecessors so successful and compelling.

Also, I believe that after the death of Mary, Michael's soul would have completely disintegrated. The guilt would have become too much and I believe he would have stayed in Sicily in his old friend Don Tommasino's house and lived out his days trying to atone for his monstrous past, with no interest, inclination or the slightest thought for what remained of the Corleone empire once Vincent Mancini took over.

Having watched III again recently (which I love just as much as I and II, although I know I'm in the minority there!) I believe that Michael's peaceful death was the perfect way to end the series. If every loose end was tied up and every question answered, imagination would surely vanish.