Why having a dedicated MT and dedicated OT is a TERRIBLE idea

[Kalise]

they try to do that all the time and never work

No they don't.

They have literally not once tried to do this.

As evidenced by the fact that PLD is literally the only Tank that has anything like Cover/Intervention that can allow them to effectively use CD's for the active tank, increasing the number of CD's a MT has by 2-3 (Cover, Intervention + Sentinel and Intervention + Rampart), heck, potentially even 4 if you end up using PoA for a TB instead of a RB.

As evidenced by the fact that Holmgang still exists with its shorter CD than any other similar skill.

masive homogenization of dps, utility and mitigation will kill tanks identity specially since we want diverse tanks kits what you ask its imposible by how the rotations and stuff works, some tank will be always superior to the others no matter what

It will not kill tank identity, since no tanks identity is "I'm the best support" or "I'm the best at cheesing TB's" - Well, no tanks identity SHOULD be those (The fact that that is what PLD/WAR are respectively is not indicative of what would happen in a world where actual balancing has been done)

DPS at the current expansion already is balanced. You look at logs and you can see that all 3 tanks are roughly equal with each other. None of them play the same, yet they all are achieving similar DPS. Maybe you could adjust some things here and there, such as how easy it is to maximize DPS on WAR because of how much their damage hinges on IR+FC spam (Which to be honest, I abhor the design of, it's neither fun nor interesting to have all your damage pushed into this 2 button combo for 10s every 90s)

A large portion of mitigation is already homogenized, thanks to Rampart being a role action and every tank having a form of ~30% DR as a CD (Vengeance, Shadow Wall, Sentinel) - Pretty much everything else is fluff outside of invulnerability skills (Which I still think shouldn't exist/work on TB's because these are the skills that are problematic to balance, especially if wanting to keep them with individual flavour)

The major things that need doing, is pulling out all of the fluff "Defensive CD's" that aren't mandatory for soaking TB's and making sure that every tank has equal mitigation for TB's. I.e. Rampart, Shadow Wall, maybe a 3rd CD (That isn't invulnerability). You can tweak things to have individual class flavour too if you wish, like maybe Shadow Wall generates Blood when attacked or maybe it's a bit stronger against Magical damage. Perhaps PLD gets a slightly more potent Rampart but with a longer CD.

Then it's a matter of making snap enmity more equal across the board. Which currently means buffing PLD's snap enmity. Which could be as easy as tweaking CoS to have a higher enmity modifier on its initial hit of damage, giving them a potent oGCD enmity boost (Akin to how WAR can Equilibrium and DRK can DA Dark Passenger)

With the final thing being to sort out utilities. It's not necessarily needing to give every tank Cover and Intervention and an AoE shield, but just giving tools to aid the party in similar ways.
For example, if TBN was 20% on allies it would be the equivalent of Intervention + Rampart, if it could scale up somehow with a CD then it could literally emulate Intervention, but in a DRK theme.
Then party DPS boosts can come in the form of many things. Like a thematic thing for WAR could be a Warcry that boosts damage or something (Instead of SiO being a random shield that has no real thematic reason to exist). DRK could get like a debuff to put on the boss to increase damage taken. While PLD gets to Divine Fail for extra LB generation or something.

idk if you see this as a dev perspective, but having to balance against 3 its much more harder that balanced against 1, its save a lot of time and resources since tanks are with healers the most hardest roles to balance.

But the thing is, that it just isn't hard to balance multiple classes against each other. Not in a role where everyone needs to be able to do the same thing (Mitigate TB's, Generate Enmity, Have some use while not being the active tank)

It's only hard when you start to try and create "Unique" versions of what should be baseline tools for the role.

Like, all tanks need to be able to soak TB's. But then you make a unique skill like Holmgang that just ignores them on a shorter CD than anyone else's version and you break Tank balance.

All tanks need to be able to generate snap enmity so to be able to pull without DPS overtaking with their alpha strike (Or having to wait X seconds before they can start DPSing). You make a tank like PLD that has garbage upfront enmity and you break tank balance.

All tanks need to be useful while not being the active tank. You create skills like Cover and Intervention and you break tank balance.

It's honestly not hard to see the major flaws present in current tank balance. None of them stem from the need to have specifically categorized "MT" and "OT" sub-roles.