Sorry, I only had about 10 minutes to make all my comments before work and rushed my description there. My idea is still entirely spitball--I only just made it up then and there--but I think that effect (like a passive mass-reverse-Shadewalker), practically speaking, would be about right.
Think of it as some 20% of all non-tank enmity as dormant, latent, inactive. Whenever a tank hits, however, they instead claim that latent enmity up to a total of 25% of their own dealt. Thus, if, say, a tank and another ally were putting out the exact same nominal enmity continuously, the tank's enmity throughput would be 50% higher. (80% vs. 100% + 25% of the 80% --> 80% vs. 120% --> gap of 40% --> 40 is 50% of 80.) This would assume a removal of (enmity-based) "tank stances", though, or else the gap would have to be reduced significantly without said tank stance. These particular numbers are just spitball examples anyways; a more appropriate gap would be around 33%, or would involve applying this mechanic only to enmity skills.
It's poor bandaid fix even then, but I believe something like that would do the job. Diversion's enmity loss (though, to a much reduced degree) should honestly be the default, with the ability being replaced instead with a situational to massively enhance enmity generation. That would allow you to kite or redirect mobs, even when working with tanks, all while giving tanks a larger enmity pool to work with.
That said, in regards to the disproportionate and weirdly tiered Attack Power growth over ilvl, specifically... personally, I'd rather see tanks just get appropriate, capped attack power rewards on their accessories as not to need pentamelds. Ideally, you'd revise Vitality to grant, say, .33 Attack Power per stat, while Strength would give .66, rather than the more even AP split that therefore fully favored Vitality back in Heavensward. From there, you'd include two-thirds each of the normal primary stat value of Striking/Aiming/Healing/Casting accessories of the same ilvl to Vitality and Strength and give those accessories a shared cap of 167% same-ilvl non-Fending accessories' primary stat total, offering the choice of Strength or Vitality. Or give 75% of each and cap it there. It doesn't need to keep up completely; it just shouldn't feel awkward in growth progression, nor restrictively expensive to BiS.