Hopefully, GF3 WAS the definite final, and I bet FFC had Michael suffer the ultimate punishment of seeing his daughter murdered, and then drop dead at the end, to assure no more sequels; it was a message to the audience, "this is it - full cycle - the end".
I don't think anyone would care how the family carried on with Vincent at the helm (well, I wouldn't, anyway). Without Vito and Michael, Sonny and even Fredo there is no more story. The next generation of Corleones would be about as interesting as the next generation of Kennedy's. Any plot dreamed up for a GF4 would be just another mafia story, especially without Puzo around to contribute. I prefer and hope we let the classics (GF and GF2) stand on their own.
Apples