Personally for my take on what I'd expect with PLD would be skills in synergy with the current set ... and an improved ability to expend MP on Stoneskin for self-mitigation before large hits:

Skill:
Indomitable Will -- 40-60 second cooldown, for 10 seconds casting cannot be interrupted by damage (enough time to pop a cooldown + soften the first hit with SS, and an after-hit).

Trait:
Hardened Will -- Permanently reduces the chance of damage interrupting casting.
Spirit Drain -- Spirits Within now additionally leaches MP on each use.