I think people who are looking at major changes to PLD are taking the imbalances too far. PLD doesn't need any huge design changes in my opinion.

You start by looking at the primary reasons why DRKs and WARs are desired in progression content. It's really an issue with raid DPS, raid mitigation, and comp synergy.

So, raid DPS. You need to increase PLD's contributed raid DPS. Pretty straight forward. If you don't want to increase their individual DPS because of design flavor, then you need to increase their indirect contributions through raid support. I don't think it's effective or easily quantifiable to base this around the idea of PLD survivability increasing healer DPS. Also, whatever change you make has to have a larger impact on PLD MT DPS than their OT DPS because PLD OT DPS is not that far behind WAR and DRK.

Personally, I think shield swipe is an easy target for killing five birds with one stone and way too good of a candidate-for-change to ignore. First off, they need to readjust the skill anyways because it wasn't properly adjusted for HW. Secondly, any change you make to it will be inherently skewed towards MT DPS since you need to block an attack to trigger it. Third, it reinforces PLD's unique characteristic of having a shield. As is, you block an attack and don't really care. PLD's shield should mean something. Imagine getting excited about blocking an attack because you look forward to shield swiping. It also makes tanking more interactive with more reactionary skills. While pacification is a unique debuff it's clunky in application because of how selective its effect is and is largely a useless effect especially in raiding content where PLD is struggling. So, it's an easy debuff to change. Lastly, using Swipe more helps PLD's TP issues. It's the final skill you get from the gladiator quest line. It should feel like more.

As for PLD's raid DPS contribution in AoE heavy fights, if you don't want PLDs dealing actual damage because of design flavor reasons, then just change the debuff for Flash's trait to something more meaningful than blind.

Next, raid mitigation. This is another easy issue to address because there is one skill that is obviously under performing for this function -- Divine Veil. WARs have Storm's Path. If you look at the debuff from a different perspective, it's essentially a persistent damage shield equal to 10% of the incoming damage with really high up-time. DRKs have Reprisal and Delirium which are both similar to Path. Divine Veil is a stronger shield but with a tiny fraction of the up-time that Path, Reprisal, and Delirium have. The biggest advantage DV has is that it doesn't require a full combo or parry to apply but even that is complicated by requiring a heal to trigger.

In the end, you have the sworn protector of the people lagging behind DRK and WAR in their ability to protect their allies. If you want PLDs to be the defensive tank, then make them the defensive tank. Give them raid mitigation that is actually better than WAR and DRK. Give them raid mitigation strong enough to cover for WARs full DPSing instead of applying Path at a DPS loss, DRKs full DPSing with no need to upkeep Delirium, raid groups bringing NIN / DRG, etc.