I agree with Goombah, but I think you can add some more to that cast that makes it really stand out.

To me, if you wanted to make the third one, Coppolla and Puzo could have reached back into the original book and pull out the Maranzalla war. With this, you have the reason others feared Luca Brasi, the potential rise of Sonny, the reason Carlo was "kept in the dark" and more relevance for the consigliere role with the contrast of Hagen (and I mean, the third would have to have Duvall) and Genco.

You could have kept some of the plot from 3 and include these bits, rather than rely on the name of the actors, the director and the writer of the screenplay.

My (humble) opinion.