Yes! Michael forever! We don't see half we could about him! In the book his relationship with Kay goes differently all the way! There's almost nothing left in the film from their romance! And her parents too. And her leaving him, talking to Hagen and returning...
Also after his daughter was dead, Michael could not retire to Sicily immediately. He had to handle that immobiliare business and teach Vincent, who is not as good in understanding business as guns, and doesn't make yet a succeedor Michael requires.
I wonder, will Tony after his sister was shot stop playing and singing a fool, realise that he is a normal full-size Corleone, kill someone in revenge and start helping his father and cousin in family business?