You can't "maintain" balance that isn't there. PLD is pretty clearly the distant third choice for 95% of content.
PLD's problem is much the same as MNK's problem. It's a specialised job whose specialisation does not serve a purpose or bring a net benefit to the party. They want it to be a "defensive" tank but not only is it not even the best in the game at what it's meant to specialise in (hello WAR), the few in which it could use its toolkit to the fullest are never utilized in-game (ie, places where you can abuse Sheltron). It's a fundamental design flaw, or rather a conscious design choice, that holds PLD back.
I'm not saying PLD's toolkit would be great even if the devs were more generous in providing circumstances where it could succeed, because its toolkit is definitely critically flawed in a variety of ways compared to WAR (and probably DRK but I don't play DRK). To get the dumb rebuttals out of the way early; **no, PLD does not take "way less" damage than the other two in any fight** (it will in fact take more damage than the other two in magic fights and marginally less in physical ones by virtue of Sheltron). And no, PLD does not need more raw DPS; it already is arguably the best single-target DPS of the three tanks. The problem lies with how it achieves that DPS, and that much like MNK, topping DPS meters doesn't matter when you're pulling the raid DPS down by being there.
Some of the key problems some of which are dungeon-specific, some raid-specific, and some apply to both:
* Clear lack of any raid utility whatsoever. Not a deal-breaker on its own but it's a back breaker when combined with the rest of this list. It doesn't increase raid DPS, it doesn't decrease incoming raid damage. WAR and DRK can both do at least one of those things (and WAR can do both).
* Absolutely inadequate crowd control. It is, by a country mile, the worst tank for Duty Finder dungeons because of this. If it could compensate by speeding up boss kills it wouldn't be as annoying, but it can't.
* No synergy with other tanks. DRKs want to MT because of Reprisal, but they want a WAR OT because of Storm's Path/Eye. PLD and WAR both want to OT but WAR wants to pull for burst, but WAR's DPS combo doesn't have a threat multiplier. PLD's DPS combo has a threat multiplier in it which makes tank swapping a pain in the ass with WAR.
* A clunky toolkit marred by useless skills. Every job has useless skills (hello, One Ilm Punch and Haymaker), PLD's just sting even more because the entire kit is a disjointed mess. It's got one enmity combo, one DPS combo that has an enmity modifier in it anyway, and its best DPS combo can only be used once every 24 seconds because the ender is a DoT. That combo also happens to be the one that restores MP (but not enough MP) so if you're ever in a situation where you need MP (group pulls usually) you have to completely destroy your DPS to recover it. Sentinel's cooldown is brutal, Bulwark on the whole is a worthless skill, Tempered Will is so incredibly niche in its utility that it's borderline useless and also has a stupid cooldown. Divine Veil has a ridiculous trigger for no discernible reason. And then there's Clemency, or as I like to refer to it, "the slap in the face that hits me every time I remember they made Clemency at the same time they made Equilibrium." Cover is... Cover.
* I know some people are going to want to fight me about Bulwark so let me just address it now. It's a terrible skill in general. Completely worthless in raids because a CHANCE to block is not good enough to guarantee surviving a tank buster, and only slightly less worthless in dungeons where it's the skill you pop with Foresight once you've burned every other ability you have because your team took too long to kill the group you pulled and you're about to die. If it had a shorter cooldown it might be at least a serviceable skill in dungeons.
* PLD is punished by the cross-class system. This may or may not change with the combat system changes, but as things currently stand PLD not only benefits the least from the cross-class system, it is actually punished by it. PLD has access to the least useful skills, but other tanks get free access to Provoke, meaning other tanks essentially get an extra job-specific skill for free because they don't have a native Provoke equivalent they are forced to learn. This is a major issue.
* Clunky and uninspired stance system. PLD's stance system is mechanically clunky compared to DRK and uninspired compared to WAR. A lot of problems with PLD's kit could be remedied if Sword and Shield Oath augmented or enhanced other abilities.
There are a number of things they could, and should, do to make PLD a more attractive option in both Raids and Dungeons:
* The most obvious is that it needs better crowd control and a way to contribute to raid DPS. I'm not keen on just giving it flat AoE damage, I'd rather they do something with Flash and Circle of Scorn. Make one of them increase enemy damage taken or something. Lord knows the Blind effect of Flash isn't doing anything.
* Make Clemency not an insult. The fact that they thought up and implemented this skill at the same time as they made Equilibrium absolutely blows my mind. Clemency is inferior in every way. Leave the MP cost, double the potency, make it instant. At least then it might actually help you or your healer every once in a blue moon.
* Get rid of the curing requirement on Divine Veil. It's just fucking stupid.
* Increase the amount of MP Riot Blade restores. Or provide another way to restore MP faster. Or ideally both if PLD is going to get more skills that use MP. Having to spam Riot Blade for MP is a major pain in the ass and an even bigger DPS loss.
* Cut the cooldowns of Bulwark and Sentinel. Sentinel's by 30 seconds, Bulwark's by 60. Bulwark is a useless skill in raids and on a shorter cooldown it would at least be sort of useful in dungeons occasionally.
* Do something about PLD's combo chains. I'm not really sure what the best solution would be, but something needs to be done about where the threat modifiers in these chains are to make tank swapping less painful. Making Goring Blade a normal action instead of a DoT would probably be a good start.
* Improve the Oath stance system. Bare minimum, stance swaps should probably be instant. Ideally they would add some kind of stack mechanic or ability augmentation PLD can leverage to address some of the problems outlined above.
* Do something to make Cover less shitty. Add some kind of enmity suppression/transfer to it, or TP regain, or anything to make it useful.
* Change the effect on Rage of Halone from STR down to a flat physical damage decrease. We have no idea how useful Stat Down effects like Halone and Dragon Kick are, I'd rather they both just decrease damage dealt by a flat amount.
* The big one... take Storm's Path away from WAR. I don't know if giving the effect to PLD (in place of the current Halone debuff, probably) is a great idea, but as long as WAR has Storm's Path, it will always be one of the two tanks you take to anything. WAR can already increase raid DPS with Storm's Eye, it shouldn't also be able to decrease raid damage so dramatically.
Above all else I hope they're more inspired/intelligent with the skills they give PLD from 60-70, because looking at what it got compared to WAR from 50-60 is pretty frustrating given the situation PLD was already in going into Heavensward.
**TL:DR** They should look at that angel boss from The Lost City of Ampador (Hard) for inspiration on how to improve PLD. A lot of its mechanics could translate very well to PLD.