Winegardner is writing it, and I read somewhere that the plot will center around the role that the Mafia played in the asassination of President Kennedy.

Now that we've had The Godfather Returns and are about to get The Revenge of the Godfather, I hear that other future titles may include

The Return of the Revenge of the Godfather
The Son of the Godfather
The Return of the Son of the Godfather
The Revenge of the Son of the Godfather
The Return of the Revenge of the Son of the Godfather
The Godfather: The Early Years
The Godfather: The Early Years Part Two