I'd say if they wanted to do it right, they would need to split the "job" into 3 classes/archtypes. Since if you give to many ability to one class/job they become to OPed, so you have to find a means of replicating a class/job like Blue Mage without making it to overpowered.

A thought I had come up with a while back is take the concept of Blue mage from FF6 and do the following.

Gau=Replicated actions of enemies, wide list of skills and stayed in said form, abilities known as "Feral". This could be used as the DD type of Blue Mage, mimicing the fierce nature of the creature to deal damage. Feral forms could be Beastkin, Scalekin and maybe Forgekin and in this concept it would work like monk except the forms wouldn't be 1-2-3 it would be more like 1-2-2-3-3-1-3-2-3-2-3-1-3 in which abilities don't really switch in a forward momentum they go back and forth based on the ability used and certain abilities could be used to reverse the process.

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Strago=One that was the Blue Mage, using magical spells learned from enemies to attack them. While the concept of learning from enemies would be semi difficult to add in game because of the nature of the way abilities work in this game, the way this version of Blue Mage could work, would be the tank class which would have 3 forms to switch between that would alter the abilities based on form used. It would work a bit like Monk except the path of stances wouldn't be linear and abilities would allow one to go back and forth between let's call it "Beastial Aetherial"

Basically the tank would have 3 forms which would range in their potency, Beastkin, Soulkin and Voidsent. Beastkin would grant increased defense but decreased attack when in said form and alter the effects of a certain set of abilities. Though using some abilities would change ones form so for the tank it would be knowing when and how to switch from form to form since the other forms would do the following.

Soulkin (Curses) In this form they are unable to switch out of it til the curse wears off (but with an ability later on they would be able to remove the curse to go into another form) In this curse form it would do something like, lower defense but all attacks drain HP, Increase attack but have no defense towards magic, increase defense but you are unable to attack etc etc. There would be either 3-4 curses that the character would be able to obtain but those curses while active would alter certain abilities so perhaps one attack ability has an additional effect when one Curse is active.

Voidsent Occurs when Soulkin effect wears off but the voidsent effect would be based on the ability you use once i takes effect, so say 3 abilities determine the voidsent ability, one could be hardened defense and alter 2-3 abilities, another regen effect that effects no abilities, and last one could be an aura DoT that effects some abiltiies. But voidsent would only be effective for one ability upon which you would switch back into beastkin and then the cycle would just continue.

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Finally there was Relm who's paintings came to life and acted a bit like summons in a sense. With Relm her "Canvas" concept could be used as the support version of Blue Mage in which players obtain empty canvas's as they level as with a tag to them like "Curative Canvas" "Support Canvas" and then in the same sense as aetherflow which is an instant active player's could obtain Scalekin, Seedkin and Vilekin instant abilities that would "draw" on the canvas and imbut it with an ability.

A Curative canvas could have an effect when you have Scalekin or Seedkin "drawing" active such as Scalekin would heal and add semi stoneskin while seedking would cure and grant a regen effect.

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Basically what I"m getting at is a Blue Mage is perfectly plausible but they can't make the same mistake like they did in 1.0 with Conjurer and THM and bloat those classes with magical skills when they could of been split into more classes and kept balanced. They could make a variety of Blue Mage concepts and keep it balanced all while not giving them to much, they've already shown that they can change the effects of certain abilities like Arcanist pets turning into primals or faeries depending on which job they are on so they could alter abilities from class to job based on which form of Blue Mage the player takes.