It has been hinted at that Puzo either outlined or wrote a screenplay for a Godfather IV movie. I know in the interviews for the DVD set that he did back in 1997 that he really wanted a fourth movie to be made.

It was written in The Godfather Companion back in 1990 that "IV" would alternate between the missing decade between the flashback scenes in II and the first movie, when Sonny came to become part of the family business and the first "gang war" transpired between the families - a time when no one could touch the Corleone family.

I wonder if Winegardner would consider doing a novel based on Puzo's outline for the 1930s-era, when Vito consolidated his power and rose to the peak of his powers and a young Sonny made his bones with his first murder. I think this could be very interesting...