disciple of magic tank!!

[Eggnook24]

Meh, have WHM new job wear full armor and make it a cleric tank. What the hell.

[KikoriL]

Blue Mage. The tank role's need for HP and Defense justifies their need to get hit by things to learn things.
Directly scan enemies to learn strengths and weaknesses (visible debuff icons to show the weakest element / damage).
Cycle defensive buffs to react to the element and type of damage incoming, cycle offensive buffs to deal damage your target is weak to.
Reflect enemy skill damage / types, reduce negative status effect duration / potency.
Temporarily store enemy-cast spells to re-cast back on the enemies. (Megaflare, anyone?)

[TheodoreMcIntyre]

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I'm not sure that monsters are weak to any one element more than the other, and I also recall reading about how elemental bonuses aren't really a thing for Player -> Monster damage.
I've always imagined a DoM tank working based on barriers, like a BLM does. Give them low HP but a tank stance that absorbs, say, 50% of damage as damage to your MP. Make it a piety based tank. More MP = more damage mitigation. Uses flashy illusions to keep hate and its AOE attack uses TP to absorb, say, 15% of damage as MP. Cooldowns would include things like an MP version of Bloodbath, something that reduces the amount of damage to your MP per hit, an MP version of invigorate, and its "Oh Shit!" button could be a barrier that absorbs damage equal to twice your max MP.
I dunno, I like to toss around the idea of out-of-the-box classes. I would like to see more unconventional class types in XIV.

[KikoriL]

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We also have nearly no practical research about resistances... except one very nice link here.
https://dl.dropboxusercontent.com/u/...%20summary.TXT
Someone stacked it up and found out just how useful the elemental resistances can be when built up. Magic users strike me as able to do the same thing with a skillset around it, without resorting to overmelding tons of Materia to do so. It stands to reason this system has a similar kind of math to the slash/pierce/blunt styles of damage, and could be turned around to allow offensive aspect damage specialization as well.
Just saying, though, it's not fully explored and seems like it could be a quite practical wildcard of a tank that certainly fits the magic bill in its own right.

[melflomil]

They should gain cross class skills from THM. Manaward (Creates a barrier that nullifies magic damage totalling up to 30% of maximum HP).
I would say manawall but job skills are specific for that job.

[Lord_Zlatan]

A priest type class that wears cloth armor, carries a shield, and uses a mace.

They could cross class from GLD and CNJ, and have their own healing and buffing magic, with debuffs coming from their combo attacks (+enmity as well)

[Thistledown]

I want to see some sort of a Morpher from FFT:A. I know it's not exactly canon having never appeared on the numbered series but being able to transform into a monster to do tanking sounds so badass. Maybe we can have a base class named Beastmaster, which specialises into two jobs Morpher (Magical Tank) and BLU.

[Dreklore]

Geomancer there you go then have it turn into a blue mage job being Geomancer 30 / Monk 15