I thought Wakka had good development and was a fun character he's definitely a favorite of mine from X.
I'd give Yuna a personality and try to make it interesting as well as definitely changing her voice actor.
If I could change more I'd make dark aeons in the american version and make the final boss at least 3x harder, which would still be too easy but I'm hoping for something possible.
A little less linear maybe, with a few little side things throughout that have impact on th story, then less dull ones at the end (such as the chocobo race, lightning dodging and other celestial weapon quests).