My theory on the matter of classes/jobs…

Thief-> Ninja (job), req: 30 THF + 15 ARC [DPS, shuriken/ACTUAL daggers for weapons, status effect attacks galore]
Gladiator-> Samurai (job), req: 30 GLA + 15 ARC [TANK, Katana/Greatswords for weapons, with additional ranged moves]
Gladiator-> Dark Knight (job), req: 30 GLA + 15 BLM [TANK, finally introducing Darkside ability]
Marauder-> Corsair (job), req: 15 MRD + 30 ARC [TANK, finally introducing ranged tank role]