I don't think this would be a good idea as it's not that plausible. The Russian Mafiya is generally known to keep to themselves and they don't really interfere with the businesses of the American mafia families as they make enough money with their own rackets.

I think it's therefore a little bit too ambitious that a Russian crime boss comes out of nowhere and tries to take over the position of the most powerful crime family in the United States. I won't really see something like that happening.

I also think that the absence of Al Pacino will be really missed just as the absence of Robert Duvall in the GF3. I would be interested to see an older and retired Michael Corleone in the role of consigliere to Vincent, just before he dies. The funeral of Michael Corleone would be an important element of the film as it would symbolize the ending of the saga.

Imo, it's best to just focus on an internal struggle in the Corleone family. Maybe from someone who felt stepped over by Michael's nepotism when he made Vincent the boss, who till then was merely a soldier in Zasa's regime. They could also involve the Rizzi brothers who want to take revenge for the death of their father as revenge is also an important theme of The Godfather.