I know this will never happen but heres to the .0000000000000001% chance that SE could do this with a future FF MMO project cause for this to work all current job classes would have to go *poof* and start over at level 1.....and yoshi said he was never gonna do that.

If you ever played Final Fantasy Tactics then you know exactly what this system works like.
Stats and Job Classes

When you create your character you have 3-4 job CLASSES to pick from. Lets say Warrior, Black Mage, and White Mage. When you gain job levels you learn new abilities and traits. Once you reach a certain job level, you can unlock new job classes. You leveled white mage to level 20 and you leveled black mage to level 20 and you leveled warrior to level 10 unlocks Red Mage. A class that uses rapier swords and can use red magic. (Mix of black and white mage spells, but not all of them.)

Warrior = default
White Mage = default
Black Mage = Default
Thief = Unlocked with Warrior level X
Monk = Unlocked when Warrior level X and Thief level Y
Red Mage = Warrior X, White Mage Y, and Black Mage Z
Dragoon = Warrior X, Monk Y
Dark Knight = Warrior X, Black Mage Y
Paladin = White Mage X, Warrior Y
Ranger = Thief X
Ninja = Thief X, Ranger Y, Monk Z

And so on and so forth.....

Class ability cross overs

You have a set of skills and traits you can equip on each class.

Skills Main:
Skills Sub:
Trait 1:
Trait 2:
Trait 3:
Trait 4:

When you see "Sub" don't get it confused with FFXI's Sub job system. There is no half level cap and stuff....

Ok so your leveling up your level 2 Warrior class and you have that level 25 white mage class already leveled up remember? so the setup will be like this. Your current class will force the Main skill slot to be set to the skills of that class. The sub will be able equip whatever other class you have leveled.

Skills Main: Rage (This is all your warrior skills that you learned.)
Skills Sub: White Magic (You chose to equip your white mage abilites.)
Trait 1: unequipped
Trait 2: unequipped
Trait 3: unequipped
Trait 4: unequipped

So now your a rank two warrior with access to rank 2 white magic. Your sub can't go higher than your main. It has to be equal or less than.

Not the best setup being that your mixing melee with mage but this is just an example. Your stats are setup to benefit a warrior class. but you can cast heals with less int and pie than a player who is main class white mage. You can still remove status debuffs and give you and your friends weaker protect and shell like buffs maybe even haste! This could be good for solo or even duo purposes. But in a raid situation it is best to prolly go melee with melee subs and mage with mage subs....but the combinations could be amazing given the endless situations.

now for traits.

As you level up job classes you unlock special traits. Lets say warrior gets double attack, and monkey grip (Hold a 2handed axe in one hand) at what ever levels. And lets say Ninja gets Duel wield. And monk gets counter. Thats 4 to go with the example.

lets go back to our ability setups and lets pretend we already leveled Warrior ninja and monk up high enough to unlock all the above traits.

Skills Main: Rage
Skills Sub: Ninjutsu (Ninja's abilities)
Trait 1: Double Attack
Trait 2: Counter
Trait 3: Monkey Grip
Trait 4: Duel Wield

You can equip from any job classes you have leveled up. Not limited to just your main and your sub like your abilities are.

Talk about customizing your character right? Monkey Grip and Duel wield look brual... Would have to find a way to balance that right? LOL

Anyways....

So your gear and weapons depend on your current main class


FF14 could still do this to a large degree.

Have the main abilities and the sub abilities limitation instead of the mix and match of all leveled classes.

they would have to redo all the abilities tho, and make them all very unique.... but thats already in the works correct?

So thats my rant.