I feel that enmity should be lost when you take damage. The lost enmity would be based on the damage taken, basically the more damage you took from an attack the more enmity lost.
This would help make PLDs wanted as they would (or should) take significantly less damage than WARs, making them a tank that starts of slow to build enmity, but looses little enmity when taking damage. The WAR will instead build enmity fast, but also looses it fast from damage taken.