Well, from what I remember from the commentary of The Godfather III on the DVD, Coppola and Puzo had an idea to do the whole flashback thing with Sonny and have the semi-modern part of the film be focused on Michael and his relationship with Vincent. Now he didn't say much about it, but I think that the main character would still be Michael. Supposedly it would lead right up to Michael giving Vincent on final piece of advice and the sitting down in his garden before dying.

I think, though, the best thing that Coppola should do if he ever decides to make a forth Godftaher is look over the boards very thoroughly...although that may make him rethink his decision...