If I were given free rein to try balancing out the tanks, I think the goals would be
-establish a strong playstyle & identity for paladin
-bring paladin and dark knight to warrior's level
-relax group composition requirements
So global changes:
-STR down and INT down are changed to damage down (for dragon kick as well). Some boss attacks slightly weakened to compensate.
-Parry stat needs to be revamped entirely, not sure what would be best at the moment
-Fending accessories also need to be changed
Paladin specific:
-Shield Swipe changed to 160 potency and combos into every paladin WS. Pacification removed. Can be used after blocking.
-Sword Oath damage lowered to compensate for Swipe changes
-Savage Blade and Halone enmity modifiers increased
-Enhanced Awareness rolled into base ability
-Enhanced Shield Swipe added at level 44, gives Shield Swipe a 20% chance to be usable when you or a party member takes magic damage.
-Sheltron activates Shield Swipe upon taking magic damage.
-Divine Veil recast to 90 seconds
-Tempered Will recast to 2 minutes, also reduces TP cost of Weapon Skills by 100% for the duration
-While in Sword Oath, Covered damage is reduced by 20%
-Clemency cast time reduced to 2.5 seconds.
-Shield Bash TP cost reduced to 40, leaves debuff that increases TP cost of Shield Bash by 165 for 15 seconds.
It's kind of a lot, but it's all centered around getting swipe procs. This lets you increase your damage by using swipe at the right time. Paladin is still weaker against magic damage than the other two tanks, but with no INT/STR debuff any more the gap isn't as bad. Paladin's physical superiority is also slightly decreased by the elimination of STR debuff.
The Cover and Tempered Will changes are there to make them more universally useful, and the Shield Bash change is to avoid a situation like A4S, where bringing a paladin means that you're forcing your DPS or other tank to do stun mechanics (or throwing away 90 TP every 30s). It still breaks combos and takes a GCD, however. Veil and Clemency are straight buffs, because they need it.
Warrior changes:
-They don't really need this, but I think it would be "neat" if Bloodbath was 30 seconds without the trait, and the Blood Shower trait made your Bloodbath heal nearby party members as well.
Dark Knight changes:
-Sole Survivor restores 200 TP as well when its requirements are met.
-Reprisal changed to: Parries the next physical attack and retaliates with a 210 potency attack that lowers the target's damage dealt by 10% for 20 seconds. 30 second recast.
-Dark Arts Power Slash modifier increased.
-Dark Arts Scourge: Decreases the target's slashing resistance by 10% for 30 seconds.
-Salted Earth: also restores (5-10 TP, haven't run numbers) every tick to party members who are standing in it.
Trying to address any lingering TP or utility issues as well as allowing drk/pld to become available without a ninja. The Reprisal change would let them mitigate physical attacks at the same frequency as paladin and warrior, if not at the same potency, and makes their raid utility less reliant on RNG (though I think current reprisal is fine).
Anyway, that's just me. What I expect will actually happen is:
-enmity modifiers increased for paladin
-potency boosted on shield swipe
-divine veil recast reduction
-royal authority potency increase
and they'll call it a day.