Let's talk about THAT cutscene from the main story (spoilers)

[Delsus]

Thancred was originally a type of Bard before we got this western 'bard' - It's more than likely Rogue since having a FF thief won't work in this game when you get down to it, they designed the game far too limited to have a 'Thief' style of class which is why outside of FFXI you hardly see an actual 'Thief' type class.

Let's look at Thief and see how it wouldn't work.

Recurring abilities:

Flee - Because we can't have movement speed+ in the game, or even instantly remove aggro from mobs, and prevent aggro for a few seconds (however the latter would need to be balanced right to avoid exploitation)

Steal - Because we can't have us taking items from mobs.

Mug - Because we can't have an upgrade to steal to inflict damage as well.

Dual Wielding daggers - Because we can't take daggers from GLA and add them to another class, although one uses daggers on GLA and we definitely cannot have dual wielding we have no classes available that can attack with both hands. we also could never have a "dual dagger" weapon type added.

And let's take FFXI for the last two:

Sneak Attack - Because we have no way to inflict more damage using positional requirements.

Trick Attack - because we can't have any way to dump some hate.

These are the basics we would need to have a thief class/job in game, the increased speed would come from the dual wield daggers if you can see through the sarcasm you will see we can easily add a thief type class/job to the game with the current game design.

[Sylve]

Dual Wielding daggers - Because we can't take daggers from GLA and add them to another class, although one uses daggers on GLA and we definitely cannot have dual wielding we have no classes available that can attack with both hands. we also could never have a "dual dagger" weapon type added.

Uhh, And what exactly is stopping them from adding Dual Daggers as a weapon set similar to Monk Fist Weapons? Gladiators can keep their single Daggers, the one weapon equip would produce 2 Daggers for a Thief/Ninja, exactly like the Monk ones do.
Related, Black Mages and White Mages both use Staves. They even use Shield Offhands together. So its not like they haven't made Class/Jobs that utilize similar weaponry to other Class/Jobs.

Flee - Because we can't have movement speed+ in the game, or even instantly remove aggro from mobs, and prevent aggro for a few seconds (however the latter would need to be balanced right to avoid exploitation)

Steal - Because we can't have us taking items from mobs.

Mug - Because we can't have an upgrade to steal to inflict damage as well.

Flee could very well be a Skill similar to the backstep that Dragoons use for dropping hate. Except instead of a jumping animaiton, it'd have your character turn and dash quickly in the opposite direction to your facing.
Could even be used as a setup for other skills.
Steal could easily be adapted into a set of debuffs, for example - Steal: Armor, causes Vulnerability for increased Damage taken. - Steal: Weapon, causes Determination Down. Possibilities everywhere.

Mug could be changed into a Trait to add Damage/Potency effects to the Steal debuffs.

You're limiting your creativity to previously released games.

And let's take FFXI for the last two:

Sneak Attack - Because we have no way to inflict more damage using positional requirements.

Trick Attack - because we can't have any way to dump some hate.

Monks and Dragoons would like a word with you.

XI is not the only place to ever draw Class/Job skills from. Nothing wrong with taking an idea from XI and then expanding or modifying it for XIV.

[Clavaat]

-snip-

Woosh. You really missed the sarcasm in that whole post.

Anyway, I'm with the school of Base Class = Rogue -> split to THF and NIN. I believe daggers will be like MNK fists, since single daggers are for PLD, although they could change this I suppose. Style looks a lot like Kite.