I think things would have gotten VERY interesting had Sonny survived. You gotta remember, in the scene where Vito is explaining to Tom why he's out as consigliere, Vito says "I never thought you a bad consigliere. I thought Santino a bad Don, rest in peace." I think that Michael would have still taken his position as Don, and God only knows the possibilities of what could happen, what with Sonny's habit of going into fitful rages. Sonny would be expecting to become Don, and when pop passes the reigns to his kid brother, I'm pretty sure the shit would hit the fan. Oh man, the possibilities!