Even if Sonny had lived, Michael still would have killed Sollozzo, gone to Sicily, met, loved, married and lost Appolonia. This loss was Michael's turning point and changed him from forceful to completely ruthless.

I think that he would have returned to the US, become a business major in college, and then taken and expanded the Vegas empire. Sonny would have ruled the East and Michael the West, and poor Fredo would have had nothing.