Finding the best combination of stats is an optimisation problem (i.e. optimise a given function subject to some constraints). Stats in this game generally fall into one of two categories: objectives and constraints. Objectives are things that you try to maximise. Crit, det, and skill speed are examples of these. When there are multiple stats like this, you tend to prioritise them based off of whichever carries the highest weight (i.e. if you can't meld crit because of stat caps, meld det/sks).

Constraints represent minimum stat thresholds that you're required to meet to avoid incurring a penalty. Accuracy is one such stat. Once you meet the minimum accuracy threshold, there's no further benefit to accumulating more. Skill speed thresholds on certain jobs can function in a similar manner. Purely "defensive" stats, such as pre-3.2 vitality, fall into this category as well, where you may need to meet a certain threshold in order to safely tank an encounter.

The problem with parry at present is that it's a constraint stat, but there's no required parry threshold to clear an encounter. As a result, tanks try to avoid it altogether. If you simply strengthen parry's defensive value and make it mandatory for tanking encounters (this might be difficult to do in practice, though, as you might just get unlucky and never see a proc), then tanks will just find the minimum threshold to tank with. It simply becomes a second accuracy stat. If you give it some offensive value in terms of counterattacks and procs, then it could function as an objective stat. The problem with this is that you would end up splitting off tank gear into "tanking" vs. "off-tanking" sets, which pushes us back towards the old MT/OT tanking setup (of course, you may end up doing this anyways, if you make parry essential).

There's also the question of where Shield Block fits into this. Do you design shields so that their contribution is relatively fixed, allowing you to shed a set amount of parry? Do you let it have some dependence/interaction with the parry stat?

Regardless of how it's implemented, though, I don't think it's going to be a terribly interesting stat, as long as it falls into one of the above patterns. I'd be much more interested in seeing gear stats that interact more directly with your skills (i.e. affecting recast and adding special effects), but these are more complex to analyse and balance.