Firstly, i dont like tanking, never been able to handle the pressure the community puts on tanks, that being said, in order to keep the job system varied, how about you SE gives us a caster tank?
Secondly, english is not my first language, so my spelling and grammar may be a little wonky so bear with me.
Thirdly, im well aware of the speculations about Blue Mage getting added to the game and it may or may not be a possible tank caster depending on what rumours you hear or read.
Here´s my idea for a caster tank.
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First of all, how about making a job based on the ff14 world and lore instead of going back to the standard classes that been around since pretty much forever in the Final Fantasy franchise?
you know, create something fresh, something new that can be used in future Final Fantasy games onward.
So here´s my take on a possible caster tank job.
A caster/mage that focuses on barriers for defense, let´s say an Eorzean one day decides to try creating something new, with a focus on barrier magic, he travels all over the land learning the arts of Scholars, White Mages and Astrologians.
Then he also starts to dabble in the other arts of magic, he starts to mix the teachings he has recieved over the years, through trial and error and many failures and small successes he finally makes as breakthrough, he manages to create barriers that absorbs most of the force from attacks aimed at him, although the barriers doesn´t absorbs all of the attack force it´s enough for him to consider it a success.
The downside to the barriers is that they continually drain MP while being active, and they also demands concentration thus making it hard to throw powerful attack magic at the enemies, as such a balance need to be maintained, the caster will be able to throw spells atleast strong enough or being aggravating enough to piss off the enemies so they will focus their attacks on the caster, spells like electric shocks or setting the enemy on fire should be aggravating enough i believe.
You might then ask, what about the MP if the barrier drains it constantly? well how about our caster having a alternative barrier spell, one that might or might not demand channeling for a set amount of time thus interrupting all your attacks and instead it absorbs the enemy attacks in such a way that the force becomes your strength/MP.
Add to this Tank Buster barriers that will only remain active for a very small time, there is probably alot more of other functions that can be added to a job like this that i havent thought of but im sure you the community can come up with some ideas of your own.