Must I self-pimp every time?
A*hem:
Final Fantasy d20
Angelsboi, you should know this!
It's got a few little details right now. How I'd do things:
Whitemage/Blackmage: Already there!
RED MAGE: Base this off of the bard. Remove their songing, and their bardic knowledge. Instead, give them the ability to cast spells in light armor without a failure chance, and a selection of "magic styles."
BLUE MAGE: As red mage, sorta. Go Pyshic Warrior for the baseline, and remove all their special abilities, but give them a 9-level progression. I'd make the blue mages either (a) execute a level check when hit with a spell they can learn. If they beat the caster level of the spell, they learn it. Or, the method I prefer (b), make a Knowledge (monsters) check when encountering a monster to see if they can learn something from it, and then use a full-round action to gain the spell. Also, give them Scan as a supernatural ability.
If AT ALL POSSIBLE, go psionics for this...the psionics system mirrors the FF magic style quite closely...(I'll be posting an article on it to FFd20 eventually)
WORLD NOTES: Magic + Technology is a must. The technology should be based on the supernatural...magitech, mako energy, the Mist, whatever. There should eventually be an airship, and the person who gives it to 'em should be named Cid. I'd have "Chemists" be the ones making the items and using the technology (item creation feats, etc.) Include frequent opportunities to replace, upgrade, and enhance weapons and armor and "accessories" (basically wondrous items)...make sure they visit progressively more expensive and better-quality scores.

Call gold "gil," and get rid of anything less than 1 gp (or have it traded for, to keep realism). Random encounters are prefered, and whether you want to go simulation or realistic is up to you (do you enter a new battle screen, or do you fight when the monsters appear, where they do?). If you want the technology to be IX-X level, keep it fairly mideaval and add in the technology only as a specialized nation or people (think of the Al Bhed, or IX's airship-town)...it's not common or practiced, but it exists.
PLOT NOTES: Something is shaking the world. The big baddie is becoming immortal/godlike. Pull a last-minute switch (the guy they have been chasing turns out to be the puppet of something bigger). Revel in high school philosophy ("What if I am but the dream of something larger?"), and make sure that each PC has a character that exists in the world, and not independant of it (no "we meet in a bar" stories...you may want to spend several sessions having the PC's enounter each other).
If your theme is destiny, think about what that means on many philosophical levels...is it a force? Who puts it in place? How does it affect the characters and the journey they go on, every step of the way? How does the villain pervert it? Can you fight it, or can you choose your path? This kind of thing may be ripe for the inclusion of an FF favorite -- complex and mind-boggling time travel.
GAME NOTES: Dungeons and random encounters are the norm. "Boss Level" encounters are generally 2-3 CR's above the what the party can handle, but make sure to give low-hp monsters some advantage. Don't shy away from "minigames"...rolling a d20 for something like poiloting a chocobo is a good thing.
Keep in mind all this is going up there eventually....I'm just a busy college student right now, but expect a few things before break ends...I'm working on the Red Mage, the Blue Mage, and the Dragoon as we speak.