I would have liked to seen more of Sonny. I realize that his murder was an necessary part of the storyline, but I would have liked to seen more interactions between him and his family, especially him and Fredo. We know he was protective of Connie and Michael, but never really got a real sense of his relationship with Fredo.
Also, I too, would like to have seen more of Young Vito and his rise to power. He was just an interesting and likeable character.
TIS