FFXII could have been more involved with the other races in particular. They had this massive multi-race world and barely touched on it, every main character bar Fran was human. Huge letdown.
FFXIII is probably the strongest candidate for not living up to potential. The concepts had potential pretty much bleeding out of the game. But then they failed to do anything 'dangerous' with the characters and that let it down a lot.
The others I feel all did what they wanted to do and were capable of doing at the time, and while I don't like some of them as much as XII/XIII, I appreciate that there are others that do and that is largely because they realised their potential in various ways.