Think we'll get any Magic Tanks?

[Gamemako]

I think transitioning Red Mage from GLA is fairly low hanging fruit.

No, not really. The only thing remotely similar to Red Mage is the use of a sword. Otherwise, you have a bunch of tank abilities which resemble Red Mage in no way and no appropriate spells. You would have no use for Rampart, Sentinel, Bulwark, Convalesce, Awareness, Tempered Will, Flash, Provoke, Shield Bash, Shield Lob, or Shield Swipe, not to mention that if spells are more powerful than spamming RoH combo, you'd never use that either. 11-14 of 17 abilities would be useless to a Red Mage, and the rest are MP regen combo and generic DPS boost.

A Mage based Tank....let's see what we can build, should be fun.
I'm going to go with a THM job to save time and because they have some interesting abilities we can leverage as a tank.

Doubtful. Manaward is extremely limited in scope with a very long cooldown, and Aetherial Manipulation is the antithesis of a tank ability. High damage has nothing to do with tanking; quite the opposite, high damage is an obstacle to tanking balance.

When you need for a tank is:

Complete Enmity suite, comprised of single-target and AoE enmity abilities as well as enmity pull (3 abilities)
Complete cooldown suite (Manaward is already like Thrill of Battle -- need ~3 others)
Appropriate baseline strength, including +1000 base HP over THM, +25% eHP boost on top of base HP boost and all other HP, +150% defense boost, and +50% magic defense boost

For enmity, you can't hijack DoTs, you can't hijack abilities that can't be readily used, and you really can't hijack anything with an interruptible cast. The closest thing you could use for AoE would be Blizzard II, and that went from unlikely to hell no in 2.1. Bind effects in generally are not acceptable because you'd simultaneously flatline enemy damage output and maintain aggro -- everything in the game would have to be made immune to the Bind effect. Pulling is simple because all of your abilities are ranged, but your spells have (and need) cast times which a tank cannot afford. You would have to completely dump on Swiftcast, a cross-class ability, and make all spells uninterruptible right from the get-go. You can't use MP as your mitigation resource due to Manaward, which already does so. You cannot make a casted spell into your mitigation ability because you'd have to completely kill your enmity generation to use it. And the last thing you would ever use to survive is a resisted debuff. Immunity hell, redundant with other jobs in your party, and generally a balance disaster.

THM's design has a brick wall at every step for a tank job. You might try to hijack with AF/UI for defensive abilities, but it's not only chewing up your enmity resource to survive, it's also stepping all over Manaward. You can't deal with interruption without making Surecast pointless, and you have an ungodly-massive stat barrier for which to account. You lack the core functions of a tank out the gate, and you lack the mechanics for adding them.

(Then again, most of the jobs will need to be made nigh-unrecognizable from the class to second job. GLA and CNJ are great examples in that they can do nothing remotely resembling anything but what they currently do. I would not be surprised if SE simply canned second jobs entirely for the original 7 classes.)

There really isn't a precedent for this outside of Paladin, who used cures to generate threat in FFXI.

That's not really a precedent, that's just the funny result of FF11's abysmal design.