I was looking at "The Godfather Trilogy" quotes and trivia on amazon.com, and was surprised to read the following:

Robert De Niro lobbied for the role of Vincent Mancini. Director Francis Ford Coppola considered it, which would have included aging Al Pacino's Michael Corleone even more, but eventually decided against the idea.

This must have been very early in production. I guess if they had set Part III in 1990, this would have been possible. I don't think I would've liked to see DeNiro play another character in the trilogy. To me, that would be too strange.

Would they have James Caan play Joey Zasa? How about if Marlon Brando played Don Altobello? Do you see my point that it wouldn't have felt right?