I think a GF 4 could be ripe for several major "reversals of form."

a) Everyone assumes Michael would be shattered, unable to function, would shuffle off to Sicily a beaten man before finally dying in pathetic isolation in 1997.

b) Everyone assumes Kay would be turned against "the family" forever, would cut off contact with Michael once and for all.

c) Everyone assumes Anthony would continue his career as an effeminate opera singer, and want nothing whatsoever to do with Michael from then on.

d) Everyone assumes Vincent has completely failed as Don, the killing of Mary is the proof.

e) Everyone assumes after pulling on her "Black Madonna" kerchief, Connie will collapse into grief the way she did after Carlo was killed.

I think in a GOOD Godfather IV plotline, ALL of those presumptions could be turned backwards:

a) Michael has seen family members killed before (even ordered it). He survived the death of Vito, the death of Sonny, the deaths of Fredo and Tom Hagen. As I note above, I think the main change in Michael would be he would give up on the idea of ever taking the family really legitimate and turn totally nihilistic and cynical -- he would say, "as long as I'm going to hell, I might as well take as many people as I can along with me."

b) I think Kay would also give up on the idea of ever escaping the Corleone legacy, she would believe she and Anthony were infected beyond redemption, and she would decide to start working for revenge in her own way.

c) I think Anthony would also be galvanized into a quest for revenge, and decide to enter the family business to achieve that revenge.

d) Vincent of course has failed personally as Don, in the death of his lover Mary, but he has not failed professionally -- in fact, he has been quite successful; all the major players conspiring to ruin the Corleones (Lucchesi, Altobello, Joey Zasa, Bishop Gilday, Keinzig "the swiss banker f$#$k") have all been killed. (Plus the Vatican deal with the Corleones has gone through.) With most of the old-time dons on "The Commission" dead after the helicopter attack, Vincent probably stands as the most powerful Don left in the country, possibly in the world. Remember, literally within 10 seconds of the shots being fired at Michael (and hitting Mary), Vincent stepped up, drew a piece, and blew away Mosca the hitman. Thus, Vincent actually did succeed in his capacity as Don -- the key Corleone Family personnel (Michael, Connie and himself) all survived, with only the loss of a couple low-level hired hands (the 'Twins'), one Corleone-affiliated Sicilian (Calo) and one 'civilian' (Mary), while virtually all the high-ranking people in the other 'families' were wiped out.

e) Connie would also be shaken up after the killing of Mary, but she would also have been proven right that the other families were going to take a shot at Michael, and the Corleone Family needed a decisive, ruthless leader at the helm. She would get over the killing of Mary fairly quickly.

I think in any GF IV, the question of "where are Connie's sons, Victor and Michael Francis" would be a key theme in the storylines, and we might also see the return of one or more of Sonny's legitimate sons.