Well,

THM Should be laterally shifted into Red Mage

CON should be Black Mage

WHM should be a new class IMO, Adding Arcanist as the official support caster.

With the mixing system, you will still be able to make them versatile in your means, but officially you would have:

WHM: Healer/Support,

BLM: AOE Damage Dealer, Utility Caster (portals etc),

RDM: Hybrid/Debuffer Caster, Rapier/Saber/Sword/Whip in one hand, crozier/scepter in the other; the RDM has always silently been the fanatical caster in the FF series so it would fit right into THM's current story.

SMN: Single Target DD, pet class, most mp efficient etc, give him orbs containing elements which control what summons he can have, kinda like materia or the zodiac crystals.

Arcanist: (Time Mage it should just become): High Support/Debuffer, Utility. Haste? Stop? Slow? etc.

For Now. BLM, WHM and RDM are essential to the FF experience and would be very simple for the dev team to put together before PS3 release.

I surmise the last 2 would be harder to implement and better suitable for an expansion.