With the launch of Stormblood

[aleph_null]

I do prefer that they avoid sub-classifications unless they plan on going whole-hog with the idea that no two jobs provide quite the same functionality or "niche"

What makes more sense is to tweaks existing tanks so that all three can be fully used in either position.

Personally I don't really mind having subclassification into different kinds of tanks, just like how there are different kinds of dps (melees with resistance down debuffs, ranged with magic/physical dps boost and mp/tp regen, casters with high personal dps and some defensive utilities). I know that ideally any tank can be good at OT and MT, and that any combination of two tanks is good for progression, but that'll be a lot harder to balance when we get more and more new jobs. Right now we have 3 tanks and 3 combinations of tanks, if we have 4 tanks we'd have 6 combinations, if we have 5 tanks we'd have 10 combinations, and so on. I think it's reasonable to have certain combinations being "bad", just like certain combinations of dps are just flat out terrible for progression (2 melees 2 casters for example, will struggle with long fight).

There is no OT "role". It's an arbitrary, player-created designation which depends entirely on how your group decides to split up the tanking responsibilities on a given fight.

"MT" and "OT" are just oversimplification of saying which tank benefits more from taking hits, due to skills that proc based on taking hits (block/parry/just taking damage) like shield swipe, reprisal and blood price. You want the tank that benefits more from taking hits to be, well, tanking the boss as much as they can, while their cds allow (with a few tank swaps to allow the other tank to use their cds as well).

Yes it's player created designation, but it's not arbitrary, it completely depends on the skillset of the jobs. It's not arbitrarily decided that war benefits less from taking damage compared to pld/drk. It's decided by their skillsets, which are at the hands of the devs.