My guesses would be:

Dark Knight: slow+heavy dps or tank.
Samurai: Tank. Like war but with random auto counter and higher crit stat.
Mechanic or Hunter: support dps (think tactics gun abilities)
Mystic Knight: elemental DPS + magic buff
Dancer: Fast & light Dps
Shaman/Geo/Green: Healer
Time Mage: Support dps with enfeebles.
Sage/Magus: Master of all Deciples of magic.
Godbert Onion Knight: Master of all deciples War
Godbert: Master of all Deciples of Hand

I really can see a Onion Knight and Sage as being a non upgradable class', with the abilities of a job. They'd have 8 or 10 slots and traits based on the combination of cross class abilities chosen. But that might be getting off topic..