Casting Duvall in GFIII would not have made it better than GFI & II but it certainly would have made it better.

I like to imagine a script where Michael reluctantly gets in the middle of a power struggle between Tom Hagan and Al Neri.

I've read many times about the script being altered due to Duvall being written out. It looks to me like his duties were split between BJ Harrison (doing the legal maneuvering) and Al Neri (the consgliere type duties, such as sending the message to Joey Zasa after the massacre).