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?

Yeah, I hear what youre saying. It would be idiotic and confusing to do something like that and Im guessing why thats why they decided not to do that.

Exactly. Having single characters "double up" on major roles would be lame. A very brief cameo or something might be OK, but not a major role.
Bad idea, and good that they didn't do it.

That's OK, DeNiro can play Vito in Part IV.

Thats right Deniro could play Vito if there was a part IV! SOme movies have used the same person to play a different character. In Once Upon A time in Mexico Danny Trejo and Cheech Marin played different characters when they characters they played in Desperado had died. It made the movie look real stupid, I dont care how right the person is for the part, note to Hollywood : DONT DO, IT GETS US MOVIE LOVERZ ANGRY and to do it for a GODFATHER film, you must be crazy!!