First you would need to have X-2's battle system. Crank it up to 1 (or whatever fastest is) and put it on "Active." Then you'd have the combat system.

Second off, you'd need XII license system. It was the only thing I liked in that game, and it seem to provide the most customization possible.

Third, you'd need VIII's Junction system. It allows for even more customization such as stat maxing.