This depends on a few things. First, what time will it focus on. In my opinion, it should follow Vincent running the family into the ground in the 80's and 90's, as well as Vito's rise in power from Fanucci's killing to Connie's wedding. I'd love the see him make the offer the band leader cant refuse, see Luca Brasi in action, see the kids grow up and go thier own ways, etc. This would complete the Epic story, from the beginning (Vito's migration and rise to power) to the end (the family's collapse under Vincent). I think the main focus should be on the old days, considering this is the more interesting story, and that Vincent only appeared in Part 3, while the other story would tie more directly into the entire trilogy.

Also, aside from the plot, Coppola would have to direct.

Deniro would have to play Vito, Garcia would obviously have to be Vincent, and please no Leonardo DiCaprio as Sonny crap.