As the original poster, thought I'd drop in to give my updated thoughts on my originally proposed idea based on what everyone has said.

Originally I had intended the proposed Red Mage tank to be a Red Mage/Rune Fencer or Mystic Knight hybrid.
Mainly I just wanted a different style of tank that was based more off of magic than just being a walking colossus sheathed in metal.

I personally feel that what I want could easily be covered by a Runic Knight (based off Celes from FF6)/ Mystic Knight (FF5) class and this would most likely be the best answer to a magic tank.
I could see this class' relying on parry vs. PLD's block, utilizing some sort of magic shield ability that makes damage taken detract from MP instead of HP and a "Runic Blade" ability that mitigates damage and transfers X% of damage taken into MP, thus fueling the magic shield ability. There would be of course other abilities to flesh out and balance the class, but hopefully you get the idea.

As for Red Mage, I still don't feel that a generalist role works for a trinity based mmo game, even if it is classically what the RDM was. I do agree with the idea that RDM should probably be a damage dealer with some support abilities similar to the BRD, but maybe make them the melee version with armor/def more inline with LNC/DRC since they need to be up close and personal and also since RDM could wear heavier armors in the classic FF games, although not the heaviest/best.

I also like the idea that both Runic/Mystic Knight and Red Mage would be Jobs based off a Fencer class that utilizes dueling swords, maybe even with an off-hand parrying dagger that increases Parry chance similar to Shields and block.

Anyways, that is where I am at the moment. Thanks everyone for your ideas and feedback.