Stone/s! in my shoe, among others -

1. No Tom Hagen [no brainer]
2. Weak and stupid Michael
3. Sonny's or even Connie's son/s as power struggle schemers instead of inventing Vincent

I felt Sofia was alright especially compared to the performances of quite a few of the seasoned actors....

"The studio lowballed Robert Duvall" indeed

Robert Duvall felt the pay gap between Al Pacino and him was unacceptable
Robert Duvall-Seth Meyers interview

Al Pacino on The Godfather: Part III (1990)

Al Pacino - Biography - IMDb
under 'Personal Quotes' -

You know what the problem with that film is? The real problem?

Nobody wants to see Michael have retribution and feel guilty. That's not who he is. In the other scripts, in Michael's mind he is avenging his family and saving them.

Michael never thinks of himself as a gangster - not as a child, not while he is one and not afterward. That is not the image he has of himself.

SPOILER

Michael has this code: he lives by something that makes audiences respond. But once he goes away from that and starts crying over coffins, making confessions and feeling remorse, it isn't right.

I applaud [Francis Ford Coppola] for trying to get to that, but Michael is so frozen in that image.

There is in him a deep feeling of having betrayed his mother by killing his brother. That was a mistake. And we are ruled by these mistakes in life as time goes on. He was wrong.

SPOILER