Roles for Samurai, Red Mage, Dancer

[Arrik]

So a tank can definitely be a kind of mage. But one cannot work so well (In this game specifically) as a caster. At least not without making some significant exceptions to how it casts spells. There's just no way it can be feasibly done unless you do the following:

-The caster tank cannot be interrupted in their spellcasting.
-They can move while casting magic with no reduction to their mobility.
OR
-Their cast times are incredibly short (think BRD/MCH level, about the same as the GCD).

We have already seen the problems when it comes to Paladin's Clemency. They already had to significantly reduce the cast time, and even now it's challenging to find windows you can squeeze out a cast without being interrupted or having to move. Tanks in this game sometimes have to move often, and a caster that has to move often significantly drops in efficiency. Tanks also often have to tank a lot of enemies at once, especially with dungeon speed runs calling for huge pulls. More enemies = more chances to be interrupted. So this is why a caster tank needs to have these kinds of exceptions made to their casting to make it work.

Paladin's issue with Clemency would almost go away if blocking an attack prevented an interruption like in 11. That being said insta-cast spells, auras, channeling abilities could all play some role in how a caster tank could deal with this. Or if you wanna cheese it give them Aquaveil buff that would pretty much do bullet #1 here. Casting time doesn't necessarily need to mean exactly like BLM or exactly like BRD and that's it.

At that point though, are you really playing a caster anymore however? You're fighting in melee range, and basically just waiting for your attacks to go off. Tanks often have to grab enmity quickly and react fast to pick up adds or enemies who might have gotten pulled away. If the majority of your moves have a cast bar it's counter-productive to things like snap-aggro, we saw this in 2.0 Warrior's Overpower which had a long windup before going off, in 2.1 it was changed to the fairly instant version of Overpower we have now because it was easy to miss with such a slow AoE enmity tool.

Aura that keeps a mob 10' out (the caster's life could be tied to said aura so they aren't invulnerable) or whatever as long as its in a place it could normally go (not pushing Titan off the platform for example). I don't see why you seem to exclude insta-cast for a caster, it'd be pretty easy to give them the standard Flash/Unmend equivalents as spells, and even in 11 some spells were pretty damn quick. Probably a better way to do this I'm not thinking of and I could see this taking the most time to debug.

There's other things to consider too like armor types, will it wear mage robes? Or Tank gear? Is it a good idea to have a tank compete with gear with dps? (Generally not so good of an idea). If it wears tank gear do they need to add magic stats to tank gear? That's more work. How will it handle mechanics in fights where enemies are immune to magic damage such as in the Void Ark if all it does is magic damage? Or fights where magic is reflected? Why would people want a tank that takes extra damage from a boss?

Ilvl 240 with a 245 weapon PLD is at 1399 VIT and a BLM is at 918 VIT and 1253 INT. 1399-918=481 481/1253 ~= 39%. Wear the caster gear and trait them to add 40% of their INT to VIT. Ragnarok 2 did similar when switching from Cleric to Monk, there was a trait that messed with int vs att so you didn't have to restat to go from Cleric to Monk at 25 (at that point both str and int helped att). Defensive differences could be fixed with pre-pull buffs, increased amount of mitigation tools, playing with cooldowns, etc.

It would be a challenge for sure, but that's not such a bad thing.