A small suggestion: For this "Dark World" idea you mentioned...you can have all the regular classes transform to their dark counterparts.

Fighter/Knight: Dark Fighter/Dark Knight: Uses Dark Swords instead of Knight Swords, Knight has level 1-3 Black Magic. (Mystic Knight FF3j sprites?)

Thief/Ninja: Rogue/Overlord: Pretty similar to the Thief, but more of a preference to knives. Overlord has levels 1-4 Black Magic. (Kinda stumped in terms of sprite use...something dark elf-ish)

Monk/Master: Savage/Berserker: Ferocious guy. Can use Claws to aid his bare-handed attack. (Try the Viking FF3j sprite)

Red Mage/Red Wizard: Dark Mage/Dark Wizard: Forgot some of the higher White-level Magics and dabbled more in Black Magic. Mage can learn up to Level 6 Black Magic, but only Level 3 White Magic. Wizard can go to level 8 Black Magic (he can't use NUKE) and level 4 White Magic.

White Mage/White Wizard: Alright, it's gonna be kind of hard to convert a pure White Mage into Dark form. How about a fighter/mage that focuses on taking life from enemies and giving it to allies? That's all I can drop.

Black Mage/Black Wizard: He's dark enough.