Foresight says hi! ... j/k
Here is what I think about tank balance personally. Apologizing before hand for the wall of text!
Warrior:
WAR is by far the most well designed class in the game.
It started really bad in 2.0 and to fix it, SE looked at every ability they had.
In 2.x WAR lacked an offensive stance and it was kind of funny and made no sense that they lost their highest potency attacks when they tried to DPS instead of tank. People started juggling Defiance + Unchained to increase their DPS, which put stress on their aggro. Ah the times I ripped hate on Nael or Bahamut as an OT without even using BB. lol.
WAR's CD structure is unique and different from PLD and DRK in that it is mainly comprised of 2 minute CDs with some minor CDs on 1.5 minute timers and the rechargeable Inner Beast. The biggest CD in WAR's arsenal being Holmgang is at a 3 minute timer. WAR does not follow the same 90s > 180s > eternity CD structure of PLD and DRK.
WAR's toolkit screams MT and burst mitigation. ToB says I get 20% more HP now so the next attack won't kill me! Inner Beast heals HP and reduces damage further insuring you won't die and Vengeance is the best defensive CD in terms of power and duration (30% for 15s) vs CD (120s only). It was relegated as OT because of the 2.0 stigma. Though it's kinda funny as it was THE tank without a DPS stance.
Heavensward came out and SE looked at each of the two tanks and what they were missing and gave each abilities they were hurting for.
Deliverance (Destroyer in JP, I like that name better! Lol) was what WAR was "missing".
SE probably slightly overdid it with Fell Cleave and Equilibrium. But the result is a tank with a complete set of actions that all have proper and great use.
In the end, WAR's toolkit is too synergistic with itself that any two actions taken affect each other positively. Any two CDs used boost each other massively. Heck, even crossclassed CDs complement the WAR's toolkit at crazy levels (Awareness + RI or ToB + Convalescence).
Paladin:
PLD is kind of the opposite of WAR. It was the "trash tank" in 1.x series because of magic damage being rampant (sounds familiar?) and attacks that couldn't be mitigated. Making the pure high HP tank better.
A lot of PLDs abilities are residual abilities from 1.x and not in a good way. (Tempered Will, I'm looking at you).
In 2.0 PLD became the go to tank because mitigation became more important that large HP pools and bosses were mostly physical, so shield block became valuable.
PLD abilities (all of them) are really good in that they successfully do what they claim to do. There are no conflicting abilities (unless you pop a damage reduction CD during Hallowed Ground).
PLD's situational abilities themselves are good. The problem is SE can't design encounters that rely on those situations for those abilities to be used. Otherwise you are stuck with PLD if WAR/DRK have no equivalent and we have a whole different sea of imbalanced content. Those abilities end up shelved and forgotten. Not because they are bad, but there is no scenario where they are "needed".
Some of these actions are also traditional FF standards and would be a shame to not have them. The Knight always had Cover for example.
What SE could do is add niche benefits to them. Like in T13 and how cover made Earth Shaker drop no puddle, so you can put it on a melee and they would not have to run away, increasing their uptime and preventing DPS loss.
Also for the entirety of 2.x, PLD suffered TP issues. Those were slightly mitigated by using Shield Swipe, which was a slight DPS gain for a slight aggro loss. Shield Swipe was out of option as an OT... Unless you Cover the MT and pop Bulwark for funzies.
PLD OT with Sword Oath did comparable DPS to WAR DPS out of Defiance if the slashing debuff is applied.
With 3.0 coming out, SE gave the PLD additional combo finishers (something they were hurting for) along with Sheltron, Clemency and Divine Veil which added reliability to block and additional utility, great changes, but lots of clunk. PLD TP issues were not addressed. Shield Swipe became a DPS loss. Additional combos means PLD will perform its aggro combo less, this gives an underlying aggro issue.
In terms of DPS, PLD has the lowest MT and OT DPS of the three tanks. Which is fine, since they are considered the defensive-utility tank. PLD also has close to 0 AoE DPS, something that baffled me for the entirety of 2.x. That was one of the first reasons that made me want to try WAR instead. PLD has decent burst utilizing Fight or Flight but due to lack of instant high potency skills, it is still behind WAR. PLD MT DPS is hindered by its lower aggro. PLD is the only tank that is forced to use its weakest potency combo to generate aggro, which also happens to have the lowest multiplier.
So what is wrong with PLD?
Nothing really. The class, on its own, is fine. The problem lies in the current meta. With Alexander Savage, the end-game raid tier that is gated solely behind DPS checks. To top it off, all tank busters are magical. While PLD is still ahead in terms of mitigated damage, a lot of people do not feel it justifies their lack of overall DPS. But SE cannot design content where only PLD can mitigate or the other tanks will be in a worse situation than the current PLD.
What does the PLD need then?
PLD needs enmity multiplier fixes and major QoL changes. It needs fixes to its TP issues (as MT or OT). The class also needs to get rid of the "clunk" in their utility.
Savage Blade is 3x and RoH is 5x. This is 0.5x less than their equivalents on the other classes. It was understandable when it was their only combo in 2.x, but this is not 2.x. Although PLD is the only tank of the three that in its full rotation it uses 2 SBs and 1 RoH. It also includes CoS in its single target which has a multiplier of 3x. The class is still behind the other tanks in term of enmity. SE already said they are looking into this issue (tanks aggro in general), so we will see.
Divine Veil being a major one. If it's such a big CD, why does it not simply apply the shield on use? Why does it need a trigger in the first place? If it must have a trigger, then its CD should be reduced and the ability should be trigger-able by the PLD himself.
Clemency is a good spell. But PLD cannot really utilize this without coordination with the healers. Which is also fine. But as a tank, shouldn't the PLD get a trait to prevent interrupted casts? And if we want to say it's an OT spell, the cast is kind of long and interrupts combos, by the time you get to it the healer would have already got you "covered". (no pun intended)
IF PLD needs to get any changes, the first change is aggro multipliers on their main aggro combo. PLD also needs removed clunk from their utility (mainly Divine Veil) and a means to restore, or otherwise manage, their TP.
Dark Knight:
From the beginning, I'm starting with what is wrong with DRK.
For starters, DRK feels very unoriginal and it's a mishmash of PLD and WAR (more PLD than WAR) with an MP minigame. DRK has the exact same basic CD structure as PLD with some of the minor CDs replaced with more minor CDs. Shadowskin is a rampart clone, Shadow Wall is a weaker Sentinel, Living Dead is the worst I-won't-die CD on a 5 minute timer. The minor CDs are Dark Mind and Dark Dance at 1 minute.
Dark Knight is in a good place only because the content allowed it. The magic heavy encounters in Alexander Savage hide the glaring issue in DRK's physical mitigation. Also DRK's ability to slightly deal more DPS than PLD. It also has Delirium which allowed the parties to drop MNK for the higher individual and raid DPS from DRG + NIN.
DRK has a very sever lack of reliable physical mitigation which will be felt if frequent physical tank busters become a thing again.
DRK's DPS is slightly ahead of PLD but is still behind WAR. DRK can keep its equivalent to Maim (Darkside) up 100% of the time as long as it has the MP to keep it. This makes DRK the class that loses the least when activating its tank stance. DRK has no actual burst other than front loading most of its oGCDs in a small window. Out of the three tanks, DRK's DPS graph is probably the most linear.
DRK also has major TP issues. It can be "managed" but at a slight DPS loss. Still better than PLD that has no means other than to floor and then stand and auto-attack target.
DRK's MP management, while adding interesting gameplay, is just a minigame that makes you feel like you're doing something when you really aren't. We barely use some of our abilities without DA. For example, we will always use Delirium if we do not have MP to DA Souleater. We save DA for C&S or Dark Mind and would not use it on Souleater, so we use Delirium instead. MP management is in no way intergral to DRK's defenses (unlike Wrath is for WAR) so it just feels like an added layer of complexity for the sake of complexity. Not that it's complicated at all. It is down to using the same branching combo over and over.
Unlike PLD, DRK's toolkit is not made to shine on its own, but in conjunction with other abilities. For example Dark Mind which reduces magical damage by 15%, becomes 30% with Dark Arts and is usually coupled with Shadowskin/Wall for added effectiveness.
Speaking of Shadow Wall, Shadow Wall is 25% less effective than its equivalent (Sentinel) at 30% vs 40% on the same obsenely long CD for no practical reason. Some would say Reprisal is there to function with it but they forget that buffs/debuffs stack multiplicatively. With Shadow Wall up, Reprisal is only 7% reduction, not 10%.
Another thing DRK suffers is button bloat. Some of its abilities are more of the same (Unleash and Abyssal Drain... ugh). =
DRK's ability list shows DRK's inner conflict! It conflicts with itself in bad ways! To list a few:
Blood Price being the well known example. DA+DD gives 20% evasion and DA+DP gives Blind, both which mean less MP recovered from Blood Price.
Grit disabling Bloodweapon is just unreasonable. Bloodweapon isn't even that great as a DPS CD.
What does the DRK need then?
Personally I think DRK requires a rework from bottom up. It is not that it's not viable, but as a class it is lacking. The glaring self-conflict and similarities to PLD are killing it for me.
On a more objective level, Dark Dance needs to be relooked into. Maybe change it so Dark Arts make it a 60% chance of parry instead of added evasion.
TP recovery or better TP management is the other thing that DRK needs. Specially with DRK's ability to floor its TP the fastest on single target.