NINJA and ROGUE confirmed

[Nalien]

Wait... Why would there need to be a Thieves Guild? Jobs don't have Guilds...

There was no Warriors Guild. The Marauders Guild sent you to investigate something, and the Warrior NPC was also there and you went under his wing.
Scholar was literally just doing research on Nym for a Marauder, you just happened to discover Scholar along the way.
Summoner was helping a researcher from the city the Scions are from.
Pugilist? "You're a good Pugilist, go help this researcher, oh and a Monk is with him".

All they need to do is something like;

Rouges Guild NPC>> Go track down this Thief that stole from us.
Thief>> Hey, why don't I teach you to be a real Thief?

Done.

The only thing stopping Thief from being introduced is making it actually work while still recognizably being Thief, though the gear the Rouge is wearing does make me doubtful as to whether they'll even bother.

Because thief was going to be the class anyway. thats kind of what you can infer from that.

No it wasn't. That quite simply does not fit with XIVs Class naming conventions. The Japanese name them after there form of combat, for the rest of us it's a generalization of what the Class is, in this case someone who uses poisons; a rouge. They do that so that they can leave Classes open to as many Jobs as possible, Rouge => Thief is no different than Thaumaturge/Gladiator/Marauder => Dark Knight, or any potential Jobs you could fit onto the current Classes. They've always gone with Jobs being the popularized core (sorry Tactics fans) Final Fantasy Jobs, they were never going to make the Thief the Class just so we could go "JUST LIKE FINAL FANTASY I HURHUR!".

Rogues are dishonest... basically thieves or con men. They have little to do with poisons or even actually killing anybody. Most classes or characters that deal with poisons are called Assassin's. WoW is the only rogue I know of that uses tons of poison.

You're really grasping at the straws of hope and I understand it. But it's called Rogue and frankly.. it really doesn't matter what it's called as long as it plays like a thief anyway.

Rouges are not strictly con men... They've used plenty of tools across the fantasy genre, commonly poisons or traps, and certainly daggers/short swords in combination with stealth.

Thief, in the fantasy setting at least, usually avoids killing completely, and the aim in most Thief based games is to pull off a job without being noticed at all. That's why some people hate such games, there is very little killing. Rouges on the other hand, don't shy away from killing when needed.