I'd want them to add party banters, like we have in FFXV, or also in games like Mass Effect or Dragon Age. That way, all the characters could have some interactions with other characters, which would be amazing. Also, I'd love them to add more dept to some of the backstories (particularly Gau) and relationships, now that they have the technology for it.
However, I wouldn't want them to change the story itself. I want more dept, but with the same story. It's just too good and beautiful to change it.