Just an idea!
Since it's extremely unlikely Godfather 4 will be made anytime in the near future. Perhaps we could write a script for Godfather 4 here! Just for kicks.
We could start by establishing a time period for the film, and making a rough outline of the story, that most of us could agree on.
Then from that loose framework, write the film! One person writes a scene, then in the next post someone writes another, etc.
Of course this disclaimer first: "This is a piece of fan fiction that is just for fun, and in no way could be close to anything Copolla and Puzo could produce." ;-)