I think your arguement holds some validity, but if the material to be addressed in another feature film is from the time period of the 20's and 30's - "the good years when they killed everyone and nobody killed them," as Mario has stated - then I think you can draw from the material Mario has already laid out and add to it.

Additionally, much of that material is common sense:

-Obviously, the Corleones would have had a large stake in bootlegging.

-The formation of the Five Families and Vito Corleone's role in their development.

-Sonny and Tom's "adopted brother" relationship and its parallel to Vito and Genco's own "adopted brother relationship.

-Don Vito and Tom's "adopted father/son" relationship and Vito's own relationship with Genco's father - who presumably would die during this period, bringing up certain feelings in Don Vito as he himself adopts Tom and saves him death at the hands of the street like the Abbandandos did for him when he first came to America.

-Why is Luca Brasi so feared? The answer can be found in the book, but was not addressed in the film. In the film, it was enough to tell the story of the band leader to establish why Luca was so tough.

-Sonny becomes aware of his father's business and wants to become involved. Sonny's godfather, Clamenza, would spend quite a bit of time teaching young Santino about "selling Olive Oil."

-What leads Don Vito to believe that at some point this young German/Irish kid would make a great Consigliere? It is merely my speculation that a part of Vito and Genco's relationship involves playing Chess together - a metaphor for strategy. Perhaps, as Tom and Sonny grow apart in certain respects and Tom becomes closer to his adoptive father, Vito and Tom spend much time playing chess with each other bonding, where Vito "sees" something in Tom.

-Presumably, many of the characters from the sequels would play roles, such as Hyman Roth, Frank Pentangelli, and Don Altobello.

So, as you can see, there is plenty of material to draw from when making another film or, as it is the case, another book. If Mark Winegardner, the author penning this novel, isn't thinking along these lines he is a fool.