Some ideas for level 90 skills that would help give tanks a bit more identity. Although these assume that the rest of the kit stays mostly the same, which isn't too likely, but I can't hope to predict what they might change to the rest of the kit so lets just assume that they remain the same as they are now.
These are all basically skills that capitalise on their burst phases, increasing both DPS and mitigation in some way.

GNB skill: Lionheart
After completing one full Continuation combo, Continuation becomes 'Lionheart' 200 potency oGCD which can be hit as many times as you can for 6s, with each hit increasing the damage of the next one by 10%.
Additional Effect: Each use of Lionheart reduces the cooldowns of Heart of Light and Heart of Stone by 5s.
(Basically extends your Continuation burst and makes the 2nd phase more hectic with double weaving instead of single weaving)

DRK skill: Blood Curse
If 5 Bloodspiller or Quietus' were used during Delirium, Delirium upgrades to Blood Curse.
Blood Curse: Duration 6s. Bloodspiller and Quietus only consume 20 Blackblood. All Weaponskills deal 5% more damage, drain 5% of your Current HP and restore 1000 MP.
Additional Effect: The cooldown of 'The Blackest Night' resets when Blood Curse expires.
(requires more resource management (reserving Blackblood for Blood Curse) and HP awareness (HP drain), but rewards you with an additional TBN and the MP to use it)

WAR skill: Exculpated Rage (recast 180s)
(extra skill, not actually an upgraded skill, because WAR doesn't need it)
Increases damage resistance by a further 20%. Reduces the recast on all Weaponskills by 1s. Only Weaponskills can be used. Duration 6s.
(A true 'berserk' skill that's active every other Inner Release, allowing for maybe 2 or 3 additional Fell Cleaves within that window. Requires good timing, use of any necessary abilities before hand, and differentiates it further from DRK.)

PLD spell: Holy Orison (2000 MP)
Clemency upgrades to Holy Orison under the effect of Requiescat. Returns to Clemency after one use.
Grants self and all party members in range a 100 potency Regen for 9s. This is increased to 200 potency to any party members under the effect of Divine Veil.
Increases the potency of your next three spells by 50%.
(basically replaces a Holy Spirit (350+175 potency) for another +175 on two Holy Spirits and +400 potency on Confiteor. only slight DPS increase (+225 in total) but breaks up the Holy spam a bit and doubles down on party mitigation.)