In Exalted, each type of Exalt is broken up into 5 different castes. There are 25 skills in the game, and each of the castes has five skills they specialize in. You can also choose 3-5 other skills as Favored abilities; they're not the skills of your caste, but you're just as good at those skills. So you could be a Battles caste Sidereal and choose Occult as a Favored skill so you can be a warrior-sorcerer. Or you could pick awareness, stealth, and survival to make yourself a ranger-type character. I find it makes customizing a character really, really easy.
(Granted, you don't have *total* freedom -- if you're a Sidereal, for example, you HAVE to have martial arts as a favored ability. But it's much less restrictive than a class system.)
So, since I'm using the Exalted engine, I wanted to keep things similar and keep it simple. After kicking it around for a while, and running things by Mad Hatter and Carrot (both of whom have more Exalted experience than me), I came up with five roles: Fighter, Thief, Expert, White Mage, and Black Mage. I think most FF characters will fit into these roles pretty easily and, with favored skills, they could add any needed abilities to their repertoire.
(As an aside, I'm not sure what to *call* the "roles". Caste obviously won't work for the setting. I thought about using the word class, but I'm not sure about that.)
Here are the skills availabe to each role:
Fighter: Awareness, Fortitude, Melee, Survival, and War.
Thief: Athletics, Larceny, Martial Arts, Stealth, and Streetwise.
Expert: Animal Empathy, Craft, Marksmanship, Pilot, and Profession.
White Mage: Lore, Performance, Socialize, Summoning, and White Magic.
Black Mage: Black Magic, Communication, Protocol, Red Magic, and Science.