It depends on wether or not you see another film as a sequel, a prequel or a seperate story altogether.

Of course Pacino & Duvall are too old now to get away with making a prequel to the current trilogy movies.

I would like to see a movie based on the story of Vito's rise to power. We see him starting out in the flashback scenes in GFII and the begining of GFI see'e him at the top of his powers.

As to who would play Vito? who knows!