Couldn't resist lol
I would overhaul all 3 of the tanking classes to make them have their niches and uniqueness.
Paladin: Holy Knight Fantasy
Main Resource: MP.
Make MP meaningful to PLD. Flash costs twice the mana, but puts a DoT on all enemies afflicted. Use MP on instant heals and hard hitting damaging moves. TP will still be there for Paladin, but it shouldn't ever run out of TP.
Primary & Passive Mitigation: Block.
Being able to block magic (holy shield) in addition to physical, block more often, block lots of stuff. Shield Oath shouldn't be reduced 20% damage, it should increase chance of block and block strength. Increased emphasis on blocking.
Secondary & Active Mitigation: Healing
Clemency would be instant cast and be used to recover health.
Utility: Additional Healing Toolkit & Raid Defense.
Esuna, Protect, Raise (in combat), Clemency on others if needed. Cover & Divine Veil. Rage of Halone reduces ALL enemy damage dealt by 5%.
Playstyle would basically be block as much as possible to build MP to be able to cast Clemency as soon as you have MP, on others or on yourself as needed. GCD would be significantly reduced (1.5 sec maybe), to give it that faster paced style as you're just swinging with a lighter 1 handed sword as opposed to a slow 2 hander. Flash would be usable in single target for the DoT, basically this is the MP management class.
Warrior: Berserker Fantasy
Main Resource: Wrath and Abandonment Stacks
It's perfect as it is, do combos to build up stacks to do your super moves. TP will still be there, and it should be the limiting factor for spamming Overpower or Fracture.
Primary & Passive Mitigation: Health & Increased Healing Received
Warriors should have the highest HP out of all the tanks. Increased Healing on them so their health bars aren't difficult to fill up. Basically they just soak damage like a sponge.
Secondary & Active Mitigation: Reduce Damage
Stuff like Inner Beast is perfect. Build stacks of wrath to do a super moves to reduce damage taken by a percentage. Basically the idea is they grit their teeth and just brace for impact and ignore the pain. No shield or magic, they are tough beans.
Utility: Raidwide Physical DPS & Raidwide Physical Damage Reduction
Basically throwing a warrior in your group is going to increase the damage of all your physical dps people. Should have shouts to increase the damage your melee dps do and to inspire them to keep fighting, letting teammates shrug off attacks too. Storm's Eye increases physical damage taken by enemy by 10%, and Storm's Path should decrease enemy physical damage dealt by 10%.
Playstyle would be how it is now. Build stacks to unleash super moves. It's a class thats in a well place and its already pretty unique. They wouldn't be so focused on life leech how they are now, but rather focused on flatly reducing damage taken and having a giant health cushion so things that would normally rekt others doesn't phase them.
Dark Knight: The Dark Soldier Fantasy
Main Resource: Health
They would sacrifice portions of their health to either hurt the enemy, or to give themselves absorption shields. MP can still play a factor as a resource, but its use would be secondary.
Primary & Passive Mitigation: Parry
Master of the sword, they can deflect swings, deflect magic. Might look weak (since parry is infamous for that), but their real power is in their secondary & active mitigation. It's a heavensward job and requires a lvl 50, so its not as easy as the other 2 tanks.
Secondary & Active Mitigation: Leech (Self-Heals) and Absorption Shields
Basically they will have massive self healing by taking the enemy's health and leeching off them. In addition to this, they have absorption shields to completely negate damage.
Utility: Raidwide Magic DPS & Raidwide Magic Damage Reduction
Basically they will make the enemy feel ill, decreasing resistance to magic damage. Magic attacks (including healer attacks), will hurt a bit more. They will also nullify enemy magic damage and reduce its effectiveness. Delirium reduces magic damage dealt by 10% and they should have something in their toolkit that increases enemy magic damage taken by 10%.
Playstyle would basically be sacrifice portions of their health to do hard hitting moves or to give themselves massive absorption shields. After sacrificing portions of their health, they should leech some off the enemy and self heal themselves.
All 3 tanks should have equal DPS and equal mitigation so they should obviously be balanced. None of them are weak in anything, some just have little bonuses that can help you here and there. Both tanks should have viable DPS options and the choice to use their main resource to go on the offense.
PLD can be taken to help tank survivability and healers HPS
WAR can be taken to help tank DPS and physical melee classes DPS.
DRK can be taken to help healer DPS and ranged magic classes DPS.
They all have different playstyles and different niches.
NOTE: These niches should be bonus, it shouldn't define the comp or the meta and you should bring whatever and play whatever you want to play.
None of this would happen of course, but I can dream lol.