I think there are all kinds of foreshadowings in the flashback scenes. Throughout them Don Vito, for instance seems to favor (or to curry favor with) Michael above his other children. He is openly affectionate to the baby Michael when he is sitting on the stoop while music plays, and in Sicily, he is always carrying michael around. We also see Sonny misbehaving during the family reunion in Sicily, and at the very end in the Flashback to GF I where Sonny gets angry at Michael for signing up with the marines, you here one of his kids in the background saying "Mommy, Daddy's fighting again --- uncle Michael." Also I think the Fredo pneumonia scene foreshadows his weakness. I am sure there are many more examples.