Multiplicative stats

AD champions have attack speed, attack damage, and critical chance to directly multiply their power. They have %Pen and %Current HP damage to indirectly (scales with their target's defense) multiply their power.

AP champions have Deathcap, Deathfire, and CDR to directly multiply their power. They have %Pen and %Current HP damage to indirectly (scales with their target's defense) multiply their power.

Tanks have HP/mitigation to increase their power multiplicatively, however building in an optimum way against both damage types is VERY impractical. They have attack speed slows to multiply their defense against auto-attacks. They have no way of multiplying their power against skills, whether physical or magic damage. Further, they have no soft counter (giant HP item) to the hard counter that %Pen offers.

Tanks need an item that gives a boatload of HP and multiplies their HP in some capacity, whether through just boosting bonus HP by 25% or by having some kind of cool interactive passive that gives them a smallish, quickly refreshing shield based on their bonus HP.