What fields do I need to learn to create something like Final Fantasy Advent Children?

There's a common belief that 3D modelers have no imagination, and that they can only model off of someone else's design. Of course that CAN'T be true for all of them. Since I'm unfamiliar with your background, and skill level, I'll suggest that you take design courses (for example, industrial, product, or transportation design). Yes, you and I can draw, but we need a foundation in design in order for our work to be functional and believable, as well as looking "cool". In addition to this, you can learn 3D modeling on the side (with apps such as Maya, or Vue Infinite, etc), or minor in it at a design school. One of the most prestigious is The Art Center School of Design in Pasadena, CA. Good luck!