I'm guessing you don't remember DF when 3.0 launched. It was nearly impossible for tanks to queue, and DPS had the easiest time for awhile.
This idea seems like something WoW does with their PLDs and I can't say it sounds interesting. I would rather see Viking as a 2h Greathammer DPS job stone applied to WAR, personally. Which probably won't happen considering their take on SCH/SMN, but hey this is just my opinion on the matter.
3 tanks or 3 healers are almost never in competition for loot in raid. Thus, they could accommodate a limitless number of VIT/STR tanks and MND healers.
Let's assume they take the time to balance this, and they consider every minute detail, like how INT potions affect this, the party buff, and all the mechanics present in the game already that affect INT. Would you really want them to add this new "mage tank," presumably BLU or RDM, which use swords and possibly shields as well, rather than just add better spell utility to PLD/DRK? What is the fascination with a tank that uses INT for damage? If RDM and BLU use swords, does this mean their swords' damage is affected by INT too? If they add this, all of a sudden we got 3 sword wielding tanks, 2 possibly with shields, that all use magic as well. So much for adding something new....in Rift... ...an ability called Arcane Ward that functions like the tank stances... "enables Block. Causes Block to derive from the Mage's Intelligence. Increases Base Health by 25%, Resistances by 35%, and Armor by 150%. Increases threat generation by 300%. Reduces damage and healing done by 20%. " They could very easily add a buff like this without changing current itemization.
How would a RDM/BLU even offer anything different? Different combo combinations? Give me an idea seriously, because pretty much every caster's mechanics revolve around MP management which is exactly what DRK is, so it sounds like you just want different animations and gimmicks. I really don't see how people miss this redundancy and not favor simply adding more utility to PLD's CNJ abilities since they already have them, and giving PLD/DRK more magic spells here and there. It would be easier for the dev team, and also give us more distinction between PLD and DRK, which is what people greatly complain about in the first place. Alternatively, would people really rather keep PLD/DRK how they are, just to facilitate an environment where a RDM/BLU tank might be considered "different?"
If we're gonna do a true "caster" tank, I think it needs to be something completely new and different, and use their hands or some kind of psychic power. Maybe Necromancer, but that really wouldn't make sense to me. Just doesn't look like a tank, but the caster-tank doesn't really make sense to me beyond PLD anyways, so whatever.
Sorta. Their oaths are spells, and they also had Stoneskin, Protect, Cure, and Raise available to them. Their CNJ spells were, and still are, pretty much useless, but I do think they should do something with that. PLD is the only true caster-based tank I can envision making it work easily considering all they'd have to do is give them a bonus to casting speed (maybe a new ability?) and give shield blocks/parry the ability to stop spell interruption. Though, idk how they'd make the potency work for Cure. With all the crazy ideas in this thread about caster tanks though, I don't think it would be nearly as difficult, and considering we got Clemency, it might not even be necessary. I think their intention with 3.0 was to move them in this direction, but it failed a bit with Clemency considering you can see that they wanted us to use it more frequently by looking at the TP values of PLD.
RDM (° ͜ʖ ͡°) and considering that RDM can cast spells, they could back off the boss when necessary. I think RDM is their most solid healer choice out of the remaining popular jobs: SAM, RDM, BLU, DNC, BST, and RNG. That's not to say they couldn't make an entirely new healer, which they might. I also think DNC makes for a really great melee support, which is currently missing. I've never really considered DNC to be a full blown healer in any game even in FFXI where it was more of a solo class that you could do small content with. Dances also seem better suited to functioning like BRD songs, giving the party an array of different buffs or giving the enemies debuffs. We also need a pure DPS ranged job, so I think RNG would fit this nicely.
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All this discussion on SAM being both a tank and DPS (Shogun/Ronin jobs) is thrilling!