I see an irreconcilable difference between personalities: Michael's hyper-controlling and Sonny's hot temper. In my imagined scenario, when gambling is legalized among the states, Michael successfully transfers the Corleones' dominance of illegal gaming and their top position in Nevada legal gaming to the new franchises. Sonny's in the background, running a diminishing suite of illegal rackets and serving as Michael's muscle. Michael tries to establish a firm line between himself and Sonny, but Sonny wants to be part of the explosively growing legal gaming business. Michael sees Sonny's reputation and image as a violent gangster as a threat to his "legitimacy." That's when the blood starts flowing.