Just throwing this out there as a hopefully balanced measure for those who like the idea of offensive parry. Rather than auto-attacking back for a fixed potency or auto-critting a following attack, etc., have it boost attack power temporarily based damage parried over the last X seconds, and drain from that bonus based on potency you deal during it. The higher the attack power is raised, the more of the bonus can be consumed per potency. Each parry's contribution is averaged into a global tick, and each tick's worth fades separately. (The unspent portion of an AP bonus from the a parry received and merged to a tick X seconds ago will fade while another parry's bonus from X-3 seconds ago will still remain.) In that way you have a particular flow of wanting to maximize reactive damage to get the most out of a great parry, but it remains roughly lenient.

Even this I'd still imagine would be better served by in-class mechanics and falls short without more ways to manually influence parry rate (both saving and spending parry chance), but it has precedent (WoW, in this case) and should serve the needs that people seem to be wanting offensive parry for without varying so wildly in contribution, especially within the same tank job.