More replay value. The game as it is gives me no reason to ever put the disc back into my PS3. A branching storyline would be cool, but that is unrealistic to ask for in a SE-RPG. I would therefore be satisfied if everything but the story was less linear.

More AI options, the ability to switch which character to control while in battle.

Branching paths in the crystarium or whatever it was called. It feels like a sphere grid with less than half of the complexity.

More side quests during the first 20 hours of the game (more than 0 can't be too hard to do).

Don't spoil your own game in the intro trailer. It shows Sash alive in a cutscene that hasn't yet happened at the point where you're lead to believe he kills himself.