Thief as a class doesnt make sense at all. Thief is meant to be a job.
Ive seen Gladiators wearing leather, equipped with a dagger and shield, and reminded me of a thief.
and your forgeting Archer is going from a DD long range shooter to a Mid range Musical player.
Gladiator -> Thief. makes sense.
also making thief a class would ruin it all. there wouldnt be no thiefs in parties since its a class, and this games best content would need to be proceeded with Jobs.
Thief cant be a class. Class = weapon, and thief has always been the main Staple Job of most final fantasies.