IMO the bigger problem with Godfather III was Robert Duvall not returning. There were many Godfather III ideas and halfwritten scripts that were thrown out over the years/decade, but perhaps the most interesting was one about Michael and Tom falling out.
Sounds far more interesting. Also Connie found out Michael killed Fredo, causing her to side with Tom against Michael. The war would also involve Michael's son, who was a US senator; iirc the idea was that Michael's son, who was clean-ish, would be killed and thus result in complete mayhem.
Copolla allegedly wanted Travolta to play Vincent.