Yeah, that's one of my pipe dreams for GF IV: After seeing Mary killed, Anthony becomes twisted and vengeful, and decides to junk his music career, go to law school, and join his father's business to get revenge on those who caused his sister's death. All of this of course over the stringent objections of ... no - his FATHER!!!!

(Subplot: KAY is the one who tells Anthony to get into the family business!! Mary's death has made Kay vengeful at the world and she wants Anthony to help her take revenge. Connie of course is in on this plot. Michael, having lost one child to mob violence, wants Anthony to stay out of the business, to have nothing to do with it.)

That could segue into Anthony taking a Tom Hagen-like role to Vincent as a smooth, polished legal advisor working along side the vengeful, mercurial Don Corleone.

Michael, as Vincent's consigliere and Don Emeritus, urges him NOT to take Anthony into the business, but Vincent sees Anthony's great legal talents and cold ruthlessness as exactly what the family needs.

Aunt Connie, whose voice now carries as much weight in the family as Michael's, tells Vincent that Anthony is perfect for bringing the family back to its former power.

Michael and Kay, now barely on speaking terms, of course have it out. "You think telling Anthony to join the family business is a favor to ME?" Michael yells.
"I don't care if it's a favor for you," she says. "I'm not doing it for you. I'm doing it for me."

So Vincent takes Anthony on as the family's top lawyer (allowing the merciful writing-out of George Hamilton) against the advice of Michael. Hmmmm....