Now what if Kakyoin did survive Stardust Crusaders?

I can see Kakyoin playing important roles in Diamond is Unbreakable along with Jotaro. He'll probably teach Josuke about the Joestars.

But I can see him pestering Jotaro about Jolyne in Stone Ocean. Pucci will be just as vengeful towards him. He could have a brush with death again against Pucci. He could probably have talked to Giorno and we know canonically Giorno is the only one who can stop Pucci in his tracks, proven both in concept and in the ASB game.

I can say things will be drastically different if Kakyoin is still in the canon picture.