Know what would be kinda interesting? An alternate timeline where the Doppio personality is actually recruited successfully by Giorno and the gang but is still in contact with Diavalo who constantly is trying to get him to kill them. This leads to Doppio telling the gang about his closeness with the "boss" and how he can somehow always communicate with him. After seeing him use things as phones and talking to the boss one night Bucciarati realizes that Doppio is the boss, but an alternate personality of him.

Some of the gang would want to kill him before he reverts to the boss' personality, while Giorno would likely want to find a way to erase the boss' personality and keep Doppio, who Trish ends up really liking and quickly starts to see him as the father she never had. Be an entirely different series of events for Part 5, and heck depending on when Doppio is recruited, Abbacchio might still be alive.

Fanfic writers, to your keyboards!