I want it. Imagine it will be very good, even a classic: then we could al go rest in peace, and Part III will be nothing more than a bad dream...
One condition: there are two timeline (so flashbacks).

1. 1930 - 1945 (but different than flashbacks in GF II, so Vito already being powerful)
Vito working with Roth, Pentageli, Genco, and the rest of the old guys + rise of Santino

2. Nothing about 1959 - 1979. Just short mention of how Hagen died.

3. 1980 - 1997. Michael being Vincent's consigliere. Vincent having a new capo besides Rocco and Neri. One of Sonny's sons (Frankie) also becoming a new capo: conflict between legitimate son (Frank) <=> bastard (Vincenzo). Anthony accidentaly coming involved when trying to protect his father.
Second to last scene: or repeating the scene of Michael's dead, or not showing it again, but just the quiet Sicily funeral afterwards. Grave next to his uncle Paolo.
Last scene (flashback): showing Vito just before the wedding of Connie. And then a voice... "I believe in America. America has made my fortune."
*Godfather theme* playing.
So the last line, being said by Bonasera would also be a summary of Vito's life.