Instead of blocks reducing damage by a fixed percent, it could be a ramp (ie a shield currently blocking a fixed 25% becomes a ramp of 75% to 25%). Where the actual Block percent is determined on that ramp is by the value of the incoming damage. PLD Attack Power determines where the ramp lines up in relation to incoming damage values. Incoming damage values are the range, 1 to infinite, with Attack Power moving the ramp across that range: the more Attack Power, the higher into damage values the ramp moves.
Essentially, the higher the value of the incoming attack, the more Attack Power is required to increase the % mitigated from it's base minimum. Since the literal value of incoming damage and iLVL-based Attack Power are considered in the blocking calculations, this would scale consistently and in equal measure to content (at a rate similar to self healing).
And how this would play out: against a hard hitting attack, the shield may only block for the lowest amount of the ramp, but on a softer attack, like an AA, the shield block will mitigate at a higher percent. Against a low level mob, solo, or unsync the shield block becomes frightfully significant (like the self-healing arsenal becomes for WAR). If the scaling was just right, progression could see improvements of 1-5% for most incoming attacks as they upgrade gear slots in iLVL, but not so against tank busters (since that damage amount would require attack power values far beyond what's capable just to budge the value off the bottom of the ramp).
If the shield blocks 25% of a 20,000 damage attack: 5k damage in one encounter; but then that same Paladin fighting a lvl5 Lady Bug outside Limsa, their block should be nullifying a lot more than 2-3 damage off of a 10 damage attack. It should block like.. ..100% of the attack.
This is the drawback of percent damage reduction. The literal amount of damage nullified is determined by the value of the enemy's attack. Self-healing isn't influenced by the enemy attack's value, it's effective HP buffer can be 50%, 100%, 120%, 200%, or more, far better than Rampart or Grit or whatever can provide. Yes, there are pros and cons to each method, but this is why WAR is working so well now post VIT/STR adjustment. Their mixed bag lets them use whichever is best suited for the moment, and to mix both at once.
Just as WAR double-dips into VIT for both tankiness and self-healing potency, Shield Blocking would now double-dip too: as VIT increases Attack Power, the ramp moves into a higher range of incoming damage values, and as PDEF/MDEF get higher, incoming damage value gets lowered, and the lower the incoming damage value, the more the shield will block for. WHM/BLM do not have the PDEF/MDEF of tanks, they would benefit from this change when fighting lower level mobs, but in 'current' content the damage values would be too high and rarely land within the Block ramp.
We've come a long way since 2.0. This would add that player-iLVL established base-line for blocking, which right now only exist for self-healing.
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tl;dr
Change arbitrary block strength from a fixed percent to a range. Strong incoming hits are blocked at low % amount (the minimum being what shield's already do), but as attacks get weaker relative to our Attack Power, the % mitigated scales up.