Believe it or not, Thief coming from Gladiator actually seems plausible in MANY ways.
First is that both Thancred and Raubahn wield Gladiator-based weapons (Dagger in the case of the former, Sword in the case of the latter).
Second is based off of the fact that Gladiator as a whole is a class notorious for hate gain. Yet most XI vets here seem to be overlooking (surprisingly enough) the one thing that Thieves in XI use quite regularly - Trick Attack! If Gladiators exist to generate hate, then Thief takes advantage of that, and with one of their job skills (most likely gained at 30, to supplement the Paladin's lack of Shield Oath at that time) re-directs said hate onto whatever class at present is tanking. SATA, the staple of any good Thief in XI, is something that naturally would be a no brainer transition from Vana'diel to Eorzea (and how the XI vets didn't think about this first is, quite frankly, beyond me).
Thirdly, remember that Yoshi-P said that he wanted to combine jobs to make a totally unique job. Well, as it turns out, Thief is capable of acting as the base in that calculation. Think - what's the one other class that's Far-Eastern, dual-wields, and has something Ninja-esque about them?
Monk.
The key point stated constantly in the Monk questlines is "Chakra", which really just a fancy name the Ala Mhigans gave to the seven vital points of the body (where aether would be the most concentrated). Ninja, whether fictional or otherwise, are masters of exploiting these seven points on the body (whether their own or someone else's). So Ninja could use these "Chakra" points to accelerate/decelerate the aetheric flow in their bodies to, say, predict an oncoming attack (Utsusemi) or make their steps lighter than a feather (Monomi). This is only a small example, but the concept still works in theory (and on paper).
From the above example, we can also add a couple of other compound jobs capable of being created from this equation: Red Mage (Black Mage + White Mage, Inquisitor-esque magic user smiting demons with holy power AND black magic), Dark Knight (Warrior + Dragoon, both struggle with an unruly power - the former being their own anger and rage, the latter being the anger and rage of an unholy fiend AKA dragon), Necromancer (Black Mage + Summoner, former dealing with abyssal powers and latter dealing with giving form to aetheric beings - both manipulating chaotic forms of aether).
P.S. This is the only post I am afforded today, so don't expect a reply very quickly.