If Sonny had lived, and Michael had still carried out the murders of Sollozzo and McCluskey, I always thought that the brothers would've made great partners. After the peace had been made, Michael could've gone back to school, gotten a business degree, and run the legitimate aspects of the business in Vegas. Sonny could've stayed in NY and kept up the original family interests.

At the wedding, we don't see Michael and Sonny together. Although Fredo and Tom stop by the table, we get the feeling that Sonny and Michael are not that close. However, when Sonny says goodbye to Michael just before he leaves to meet Sollozzo, we see real and true affection between the brothers, and I think that the two joining forces could've been an interesting story.