Well, what do you want from Parry, generally?
Do you want it to be a trade off from damage to average (not especially reliable) mitigation? To spot-mitigation or defensive / reactive control? Or do you want it to be just an aesthetic alternative to MT dps gearing? The first only needs a few adjustments at most. The second would require some actual design. The last would be essentially redundant apart from rooting one into MT and anti-physical position, though it would likely take a bit less design work than the second.
I'm personally of the opinion that Parry should just be removed or changed to something like a new stat that accomplishes the same goal of slightly freeing up healer attention but without the elements of RNG or physical niche--for instance a stat called Steadfast which increases Defense, Magic Defense, and Healing Received.
I can throw out some more ideas depending on what people actually want or expect from Parry, but for now I'd rather leave things at that.
That said, things that I don't think will help:
- Making Parry affect both rate and bonus --> Parry just becomes further unreliable, as rate is exchanged for potency, unless the stat is outright buffed.
- Buffing Parry to the point that it absolutely outweighs other secondary stats during progression, especially if only to quickly devalue after.
- Making Parry dps contributor (of an equal or greater value vs. physical, rapidly-hitting bosses than Det, Speed, or Crit). --> Tank jobs are not remotely balanced across Parry, and Parry is wasted against magic enemies and when OT; some jobs will be unable to access optimal gearing and it would be even more important for tanks to establish two ideal sets. There are ways around these issues, but I have yet to see any of the above suggestion for counter-attacks mention them.
- [Again] Letting Parry scale with other secondary stats (without adjusting the over-time/value scaling of Parry itself). --> At some point, Parry itself would provide less mitigation that adding to Crit, Det, or Speed.
Last but not least, there are only a few guards that provide useful momentum from a (stanced) parry with which to counterattack more forcefully or quickly than normal (see DRK stance, if it actually counter-attacked a downward strike with a wheeling downward cut of its own). For momentum to be passed, you must in a sense let the opposing blade catch your own and carry through (albeit in path you control); anything else merely takes the difference, and rather than gaining any sort of power, you merely find your arms numb. Similarly, neither does offsetting the enemy's weapon (be it in a parry or a strike) give a more powerful following strike; it merely increases the time necessary for them to defend against it (more closely emulated by a reduction to Evasion). I can see either as being a mechanic for a particular job, but it doesn't make sense as a rule of thumb to then attach to Parry.
__________________________________________
Edit: inb4 (a very obvious issue with Steadfast)....
Although reliable, Steadfast would require a large stat value to actually make a difference in healer actions, especially against intensive damage. Whereas Parry has a chance (though usually with two little time to be noticed and reacted to) make that difference, or not, Steadfast would be very middle-of-the-road all the time. In the end, mitigation is about healer GCDs second only to survival of tankbusters. In that sense, the only advantage of Steadfast over Parry is that it does not need to be reacted to (it acts consistently and therefore predictably) and works against Magic damage. Though of course, if considered overpowered, the Magic Defense component can always be removed.
I'd recommend against removing the Healing Received component (spitting Steadfast's value between mitigation and recovery) if only because it would otherwise become a way to cheese or save CDs on one-hit tankbusters twice as soon. Whether that is an issue or not would of course depend on the particular fight, but as 3% can often enough make the difference between life and death, especially with our current Vit tanks (for whom that mitigation's eHP bonus is indirectly greater), I don't think it would be impossible for a stat that could potentially reach over 1000 in value to reach that level of power otherwise. Moreover, I just prefer the feel of it truly taking the 'tank' role in stride, as a figure of the raid rather than a mere finite-health wall.