(This is kind of similar to when we thought DRK might be a life-tapping tank, spending life where healers would otherwise be overhealing in order to improve defense across times where healing needs would be uncomfortably intense / bolstering defense when it matters more so that healers can maintain DoTs, etc. Could be hell to heal, essentially, or an absolute breeze. In this case though it'd just be by generating, say, a parry- or parry-strength related resource or something similar but with less RNG-ness, while out of Grit, and then popping those into a sort of parry nuke when/by returning to Grit, almost like you were spending that mana not just on a stance, but on a variable shield of sorts.)
In PLD's case this stance-dancing bit came out as sort of an Oath resource, where Shield improved general damage mitigation, blocking, and could just outright make you a shield-based badass. (Think Shield Bash nukes.) The effects of either Oath would linger a bit after 'swapping', manually convertable through charging the spells themselves. While charging, Sword Oath channels a heavy on the target while building up attack speed and preparing bonus potency for your next oGCD. Overstacking the bonus potency unleashes it as a single strong cleave. Shield Oath channels a heavy on all nearby enemies while increasing block chance and reducing damage taken, drawing said enemies towards you at the end of the charge. Overstacking the bonus block chance automatically procs your block. Either charge can be rushed to reach the overstack bonus sooner by hitting the ability again while charging (spends extra mana, getting the short-term bonuses faster, but giving less of the long-term bonuses; these will instead generate / shift over time. Hitting any weaponskil would queue it for a typical 1-GCD or 2-GCD charge (max), hitting anything else twice ends the charge at that point and performs the attack, and moving of course ends the charge at that point). Charge speeds scale with (Skill) Speed, which should be a single stat with Spell Speed anyways...