The idea of Warrior increasing its max HP as his way of mitigating damage was quite fun to me in concept. Though I imagine how it would make it fairly hard to heal when the healer's spells aren't % based.
So I imagine self heal is a close second, though barrier would fit the "temp HP" idea too. But then what to make of DRK for their defensive quirk ?
Very often all types of defense can be made to fit one of the tanks, or at least several of them. I kind of liked the idea of:
PLD: Strong flat damage mitigation+blocking, and on demand powerful healing (so little to none heal attached to DPS rotation)
WAR: Buffed max HP, be it via trait and/or defensive cooldowns, and self heal during burst. (big anger contract muscles into stopping bleeding)
DRK: Self protection through dark magic, barriers and negative effects against ennemies (damage down, blind, slow and barriers on himself)
GNB: Versatile all rounder, has some healing/regen, bits of barrier, some block/parry skills.

Of course I know some of those aren't possible anymore xD