They need to get rid of that tank trait because it creates a false sense of security and creates bad tanks that don't use cds in dungeons. Reintroduce off tank stances and split the tanks into two categories:

1h weapons - PLD/GNB (they have true invuls)
-"Shield Oath" - 20% defense, -20% dmg dealt
-"Darkside" - Dmg dealt +20%, Mp decrease while active, effect ends if MP 0. MP regen from other sources, beyond your weaponskils, is negated. (oldschool DRK players know how this works.)

2h weapons - WAR/DRK (their HP cannot drop below 1)
-"Defiance" - Max HP +25%, -20% dmg dealt, Recovery via healing magic increased by 20% (2.0-4.0 version where certain weaponskills restore HP in Defiance)
-"Deliverance" - Dmg dealt +5%, Crit increase up to 10% at max gauge (10% crit gauge extended duration for a short period, if skills are executed at max gauge, even if gauge drops below 100)

The 1h tanks will play similar to DRK in 3.0 where resource management is crucial but the pay off is keeping Darkside up full time while in Shield Oath. The 2h tanks will just be like WAR, pre-ShB, where they have to manage gauge to stay between 50-100 and stance dancing to take advantage of more defense/max HP or big dmg when needed. I think this is good compromise since you get both the high defense tank and the high HP tank. All tanks will have their Tank and OT stance ogcd so no more clunky switching between the two like in 3.0-4.0 where WAR felt good to stance dance but DRK and PLD didn't. All tanks will also get Unchained as a role skill so they can remove the dmg penalty (20 sec duration, 2 min recast) while still staying in their tank stance.

Other than that I would like to see DRK move more into "Rend" skills, since WAR is FFXIV's self-heal tank. If Delirium worked more like Requisat but instead of Confiteor, DRK did Crush Armor or even Sanguine Blade. The finisher will depend on how much gauge you have at the end and give you access to several moves rather than PLD's single Confietor. It would also not feel like a broke version of Inner Release. I would also like to see Esteem reworked into Living Dead where for 10 seconds he will match your HP and take the hit for you. If he dies before the 10 secs, he will shoot out your back and grant you Walking Dead, where your health cannot drop below 1 HP for 6 seconds with non of that heal to full crap. It works just like WAR where all it does is not reduce your HP less than 1. Simple and easy to understand.