If You Were A Class/Job Designer...

[Tranquil]

-stuff about all three tanks-

This is some very good stuff here, can get behind all of this. Great suggestions. (Except not sure how I feel about a slashing debuff being tied to Scourge of all things, seeing as it is a non-comboed skill, but all things considered, it wouldn't hurt anyone, and the lack of a slashing debuff is one of the main things that hurt DRK/PLD comps, and you rationalized this one well). I especially like the eloquent solution to DD without adjusting it too much, as I feel like DRK needs a pinch more of physical mitigation that is guaranteed, if it MTs physical heavy fights and runs out of buttons to press.

Edit:

DRK: I wouldn't mind it if Reprisal was forceable without a parry proc, if it was its DA effect to not require a parry to proc. I saw someone suggest this and at least it would bring guaranteed mitigation to a raid, especially when a DRK is OT, and it is lacking in OT utility at the moment. DRK/WAR comps would be pretty devastatingly good in 100% magical fights with this, though, but it's not like a Reprisal can't be forced up as is in these kind of fights, if there's at least a tiny bit of physical damage going out.

PLD: A RoH potency increase would be good. It doesn't need to be the highest potency combo, but it's such a pain in the ass compared to Storm's Path at the moment, DPS-loss-wise. It's like, what, 80 potency lost over not RAing? Heck, 90 potency, now that RA got buffed by 10 potency. So bringing RoH up a little bit potency-wise would be nice so it didn't feel that big of a loss to put it on. (Whether or not the enmity mod is made 5.5 or kept as is would depend on what potency they would end up at, to roughly balance it while giving it a bit more oomph than it has now, maybe). I wouldn't mind RoH being 290-300 potency.

Also, I would still prefer it if Divine Veil didn't require a heal at all to trigger. It's not a big deal if the PLD is MTing as it is, cause heals will be going their way anyway, but if a PLD is OTing - a healer/fairy/PLD GCD is lost every time to trigger DV, unless AoE healing were to be required for the party as it is (which is a likely scenario to top people off, but anyway).