Well, I think it should mainly follow Vincent as the Don in the early/ late eighties.
It will have flashbacks to Micheal Corleones life prior to the Godather Part 1, in his teens.
Also, Sonnys sons Frank would somehow be involved, maybe as Underboss or Capo.
Micheal will not be directly involved in the families problems, because he is living out his life in Sicily.
Also, the film would go a bit more into detail of the other families interests.