Final Mix isn't really anything different, The english version of the game had 4 special bosses, Ice Titan, Sephiroth, Kurt Zisa, and Phantom...So the Japanese audience complained about this, thus, SE Japan came out with FM which has all the special bosses, but since that would be a waste they added some extra stuff, for example, extra scenes with Riku, a new special boss called Unknown aka Egnigmatic Man, most heartless have changed color, and the special enhanced secret movie ASAS: Deep Dive...Am I leaving anythign out?
Edit: Final Mix is not coming out anywhere out side of Japan