See the whole thing between the books and the movies is this. In the book it ends where Godfather 2 ends if I'm not mistaken where as the third film was just on film and not an installment to the book series. Which is why the proposition is open to write the next installment.
Given the age of the actors/actresses, the rights to the godfather films being owned by paramount and the success (or failure) of this new novel, I wouldn't count on a film adaptation. I am, however, really hoping to see Omerta made into a movie as it was promised. I was told Michael Imperioli is supposed to play the role of Astorre Viola and considering he did the audio tape of Omerta, that makes it more believable.