I don't think he wouldn't include the Fredo killing in a novel of PART II, as if that novel was written - a novel that described the movie's Part II, AFTER the movie was released, as it would contradict the facts a great deal. The movies never dettered too greatly from the novel (except with Lucy Mancini, which I didn't care, and Fredo, who in Puzo's novel was more like a Stalone character). I think he would develop Fredo differently, as FFC changed him quite a bit in the first movie and continued so in Part II. In Copola's vision, Fredo was figuretively and literally weak, whereas in Puzo's novel he was perceived as a strong in muscles, but light on heart. Not stupid, not TOO weak, but certainly a lot more sensitive than either Sonny or even Michael. So I suppose Fredo's development would be intriguingly different, yet it would lead to the same events and end of the character yet again (ie the Cuba scenes). But, aside from that, we could've actually got some interesting information on the Rosato Brothers, and Roth's connection to Vito etc..

I'd be certainly a lighter in weight book, as I don't think Puzo would include more material that wasn't include in the previously released movie. In short, he'd take FFC and his' PART II screenplay, adapt the modern part sequences and make them a novel.