An idea I've been toying about when it comes to tank self sustain is to keep it because it's actually fun and gives a great feeling of tank power fantasy against the odds, but not give it freely on demand like it is right now. I like the DRK model for example where it's tied behind the invuln (which is busted granted). If suddenly all the self heals for example on WAR gets only unlocked when using an invuln (holmgang I guess?), you'd end up with something similar. Pushing the idea further, you could for example keep the invuln part of the ability like it is, or even reduce it to 4-6s for very targeted segments of an enemy attack (like the 4s of mitigation on all the signature defensives like bloodwhetting that we always forget about seeing they're the least impressive part of those). Added to that you could imagine a 15-20s of massive self healing or potent defensives like bloodwhetting and whatnot. Invulns would essentially shift to "last one standing" abilities that you can only use sparingly.
To give an idea, you could imagine it along those lines:
DRK: similar to now, invuln of X seconds, with its massive healing from damage done multiplicatively.
GNB: based on MNK's RoE in pvp, compile all the damage taken and heals for a % of it at the end or when the GNB presses the resolution button.
PLD: applies a holy shield equal to a % of the damage taken, and AoE heals around for an equal amount.
WAR: idk find something unique, perhaps with some damage counters: counters a % of the damage taken and AoE heals for the same amount?
It's just random ideas. Perhaps they're good, perhaps they're bad. But the idea is that you'd get that once or twice per raid, at best, then you're back to little self sustain, just your mitigators. So yeah i'd be okay with insane seal heals and similar cousins if they only happened every 6min or so.