Vast homogenization of roles has caused this rift in tank design which has led to where we are today.
Tanks, should have specialized niches but still be just as good at their primary job, tanking.
Let's go down the list:
>DRK
Traditionally, the final fantasy dark knight is based around consuming health to deal damage, and healing itself usually through lifesteal or sustain abilities. A tank damaging itself is somewhat out of the question but they got this class fantasy right back in HW/SB. DRK pulling large packs with blood price and abyssal drain spam achieved this drain tank class fantasy.
As of right now? It's in this weird state of being a. . . shield tank? Your primary mit button is TBN, a shield with a low duration, and your other mits are lacking.
Proposal: DRK should be the sole sustain/drain tank, fitting the traditional dark knight role. You mitigation should revolve around this sustain.
>WAR
The traditional final fantasy warrior/marauder archetype has many different names and many different interpretations over the years. In ARR, WAR's primary gimmick was being the tank that does more damage, and it's primary mitigations revolving around just having a large amount of health, with some damage reduction built into your big damage buttons. As tank damage has sort of evened out (for the most part) WAR has instead now become the sustain/drain tank.
Proposal: Bring back the WAR design of just having a lot of health, increasing health, increasing external healing received, brief mitigations for using damage skills at the right time, perhaps even lean more into the parry/reflection angle. Admittedly WAR is the one I am most on the fence about how it should be changed.
>PLD
There's not much to say about PLD right now. The traditional final fantasy knight and PLD are fairly close now. PLD has retained it's identity as being the most traditional form of tank. Most amount of mitigations, mitigations all being various damage reductions, the ability to mitigate for their team, etc. It's in a good spot, but I'd knock it's sustain down a few pegs, and make fight or flight a stack-based buff similar to inner release and delirium
>GNB
GNB is basically THE damage tank right now, and it shows with its rotation being nearly as complicated as the average melee DPS. The way it plays right now is mostly fine. I am of the opinion that GNB should become the shield tank though. Base all of it's mitigations around shielding, etc. I would also just make no mercy stack-based like inner release/no mercy.
TL;DR Tanks have mostly lost their identity because of the waves of homogenization. Reworking them to carve out their niches would be far more beneficial to the game's long-term health.
TL;DR 2: my justification for fight or flight and no mercy changes are purely based around the fact that you would no longer need to worry about skill speed breakpoints and ping issues.