I'll paste something I suggested elsewhere, as it is relevant to this and was somewhat PLD-focused:
Best bet would probably be to give PLD an enmity+ trait that acts as a multiplier to damage dealt by the PLD, then scrap "+enmity" on gear and turn it into "-enmity decay" (caps @ 20%). Atonement would probably have to be redesigned if you did that, though.
In conjunction with removing the hate cap, there is the issue if building hate and somehow having hate generated by the tank decay at a slower rate than that of everyone else.

The above I feel would work best because we're not using a combat model where abilities are spammed back to back to generate aggro, and instead PLD is at the mercy of cooldown timers and hoping one crit too many doesn't put them below someone else on the hate table. Improving damage mitigation would be the other half of the issue to be resolved, but I can't really think of how to go about it with the seeming emphasis on -PDT tank gear has these days.