Yeah...
Though, if Active Mitigation was made more prevalent that would be a way to help diversify tanks somewhat, given that they all could utilize different types of Active Mitigation (Which is part of what helps other games Tanks diversify themselves)
For example:
PLD could gain more Block chances so would mitigate by blocking 20% of incoming damage. Maybe even let them block some spells too. Given they're currently the only Tank with a shield, they should have more focus on it.
WAR could gain more healing, including some percentage health healing. Maybe even some max health increases as active mitigation (Along with current health increases. Think mini-Thrill of Battle) - I imagine something akin to Black Orks in Warhammer Online where their strength was that they would constantly proc health boosts with their attacks (Though, a little less overtuned because BO's like couldn't die it often felt like xD)
DRK could get more focus on shielding. What with TBN already being a key point for them. Stuff like turning Souleater into shielding instead of healing and the like. They could even have some cool mechanics where they sacrifice a portion of their health to gain shields based on that health loss (I.e. Something like sac 10% max health, gain 15% max health as shield) allowing them the generate buffers as well as synergizing with HoTs better by being able to mitigate overhealing.
GUN could focus on damage reduction. Stuff like putting up temporary barriers that reduce incoming damage by a percentage (Kind of like Inner Beast does now).
This could then extend directly into diversity with DPS combos (Thus OT roles/DPS stances).
Such as PLD having more of their MT damage focused on Shield Swipe (Increase its potency, let blocks reduce its CD by 1 second or something) so they want to keep blocking attacks to keep their damage coming out. With DPS stance focusing on their blade attacks instead (I mean, they are called "Sword Oath" and "Shield Oath") - Could even let some of their combos differ to reflect this. I.e. Shield Oath incorporating higher enmity generating skills that smack with their shield, while DPS stance uses the current blade technique skills.
WAR could get more Upheaval style potency modifiers, adjusted slightly so it's based only on max health rather than current health. So when MT they wanna keep proccing more health increases to deal more damage. While in DPS stance they unleash their inner beast more and could get Blood for Blood style damage trade modifiers. They could also get a version of "Cover" that allows them to proc health increases on another target, giving them some utility in the OT role.
DRK already has a dichotomy within their class, which can be expanded on. Generating more MP and thus being able to use more DA's to empower their skills during Tank Stance. Meanwhile, Blood Weapon could be converted into a DPS stance for them and focus more on faster attacks and more Blood generation. With select shielding skills being converted into damage or skill speed boosts during DPS Stance.
GUN it's hard to say without any knowledge of the basis of their attacks... But at a guess their Tank stance could involve Hissatsu: Seigan style retaliation attacks, while their DPS stance focuses more on generating more ammo to use to enhance skills.