Well, I'm not basing it on any one era of FF...more a conglomorate of eras. A lil' here, a lil' there, and then allow each individual world to have it's own take.
FFI is probably the closest to D&D propper that I know of, with Fighter = Fighter, Knight = Paladin, Black Belt = Monk, Master = High level monk, Thief = Rogue, Black Mage = Wizard, White Mage = Cleric, Red Mage = Bard. The only one that doesn't fit is the Ninja, which would end up being something like a Ranger who used spells like Magic Missle or Acid Arrow instead of Magic Fang and Entangle.
I know everyone has their favorite FF's, and some are die-hard old-skoolers, and some like the new take. I hope to eventually be able to cater to all tastes, with the "generic" classes like this being closer to the D&D norm so that you can add them in for flavor, and so the individual worlds have a point of reference.
There are some existing classes that fit OK for a class....but it's still a bit of an awkward fit with some extra baggage. Especially if you want to emulate specific games...
"Knight" could mean a lot of things depending upon the FF you're talking about, for instance. With Tactics, they're equipment-breakers. With FFX's Auron, they're reducers of stats and wielders of two-handed big arse swords who hit hard but not often. You could probably describe the heroes of FFVII and VIII as Knights, even, with their variation's on Cyan's Sword Techniques.
"Monks" in FF are more than just unarmored fighters. D&D monks can't do anything like Chakra (healing some HP instantly) or Kick (hitting all foes with one attack) or even some of the supernatural powers of Sabin in FFVI (things like Aura Bolt or Spiraller come to mind...).
"Berserkers" are different than barbarians because you have control over what a barbarian does -- not so with a berserker. Berserkers also don't get movement powers or damage reduction or uncanny dodge. They take the damage and like it.
"Theives" are D&D rogues souped up. They can't sneak attack, but their speed is unparallelled, and I've never seen a rogue swipe the cions out of a pouch or the clothes off a foe in the middle of battle, either....
"Samurai" varies greatly on the FF version...you've got the Sword Techniques of Cyan from FFVI (nothing like them exist in D&D), and the draw-outs of the FF Tactics Samurai (again, nothing like them in D&D).
Shamans have those extra animal friends/punching powers that white mages never had. Wu Jens have more versatility than a black mage ever had. Red Mages are closer to D&D bards than they are to anything else (what with the combo of attack, defense, and martial skills). Time mages can't use Fireball (Oracles are largely the same thing, in the generic version), and FF Bards sing more than D&D bards, and can't cast magic.
While you can do a sort of semi-FF style using no new classes, I think that the FF classes are different enough, especially when taken in overall effect, that they definately deserve their own classes.
I mean, sure, there's a bit of similarity -- but mostly it's superficial. The gulf between Sabin and a traditional D&D Monk is at least as wide as the gulf between a D&D Monk and a D&D Fighter.
Thankfully, it's still tryin' to be modular...so if you think the OA Samurai works just fine, then you don't have to worry about it -- the only think you can possibly loose is a choice.
