There is part in the novel where Sonny says he was "waiting for you (Michael) to become my (Sonny's) right-arm." I think the dynamic of the two brothers running the Corleone Family would have made for an interesting twist to the storyline. It was quite obvious that Michael & Sonny were opposites in many ways: in demeanor, temperament, strategically, and intelligence. What if Sonny had not been killed and he ruled the Corleone Family with Michael after Vito retired or had Vito died from the Sollozzo attempt?

It’s obvious Michael only reluctantly entered the family business after Sollozzo tried to kill Vito. Let’s assume that still happened and Vito was incapacitated. Michael still had to flee to Sicily after killing McCluskey & Sollozzo. But what if none of the Five Families were able to knock off Sonny while Michael was hiding? So Michael comes home, Vito has recovered from his gunshot wounds but is no longer running the Corleone Family. Instead, a dual-power structure is in place with Vito’s two sons: Santino & Michael.

I think the first result is that Tom Hagen no longer has any power and is simply the Family's lawyer to maintain the appearance of legitimacy. As the novel explained, the aftermath of Vito’s shooting showed several differences between Sonny & Michael. Sonny was too often blood-thirsty. When reviewing the list of men to be killed, Michael questioned his brother and was told to “stay out of it.” Michael further showed his difference of opinion saying “that’s not how Pop would’ve played it” and advising Sonny to wait before killing more family enemies. I think these differences would have made for some interesting battles between the two brothers.

As we later saw in GFII, Michael lost a lot of his humanity and became so ruthless that killing a brother (Fredo) was not out of the realm of possibility. I think that over time, Michael and Sonny could have drifted further apart and a power struggle could have ensued. I believe that Michael had enough cunning in him that he "could've outfought Santino."

It would be an interesting debate as to who would have come out on top. On the one hand, Puzo mentioned that it was actually Santino who devised the plan to take out the heads of the Five Families. Yet it was Michael who actually executed the plan to perfection. Sonny's obvious weakness was his temper and I think Michael would have definitely been able to use that against Sonny. But Sonny was no dummy either and probably could have figured out a way to kill Michael if there were no other alternatives.