Parry/Dodge tank could be playing an active dodge/parry mitigation instead of it being a passive %chance. Most melees in the game (WAR, DRG, MNK) have two combos they alternate. Dodging and parrying is about reading enemy movement and predicting where from next attack would come.

It's also a bit about placing bets on where the next attack will come from and reacting even before the enemy actually executes the attack. If you like soccer, check out random video at the bottom illustrating successful and unsuccessful bets to dodge/parry enemy attacks.

My idea is for a very technical dodge/parry tank. It is by no means polished, only a base framework, but I believe this playstyle is entirely possible to be implemented into FFXIV and give some unique experience. If could be either a Samurai with Great Katana, Dark Knight with Great Sword or Bastard Sword or Ninja with Katanas/Kunai.

So here we go! The framework is as follows:
  • You have three imaginary areas of your body you actively defend: left, middle, right.
  • Enemy attacks have new imaginary property of being attack executed aiming at left, middle or right part of your body. Which side enemy attacks is random, but possibly there might be buff or ability to tempt enemy into attacking more frequently at specific side, or dissuade enemy from attacking at specific side to give you more advantage in planning your defensive and offensive abilities.
  • You have two attack combos (Left Stab/Middle Stab/Right Stab and Left Slash/Middle Slash/Right Slash), giving you a stacking defensive buff at the same time (up to 3 stacks): Dodge Buff gained from stabbing and Parry Buff gained from slashing. So you deal damage to the enemy and at the same time build up defensive stacks protecting three areas of your body.
  • To take advantage of WAR slashing debuff or DRG piercing debuff placed on enemies, the above mentioned Stab and Slash attacks could really have piercing and slashing property respectively, for extra damage+hate generated.
  • Each Dodge Buff and Parry Buff stack gives you +33% chance to Dodge or Parry respectively, applied to one of your body areas (left, middle, right).
  • Additionally, each Parry Buff you gain for body part increases your Parry damage Mitigation of that body part by +20%, stacking with natural parry damage mitigation (static +20% for all jobs in game) of the whole body up to total of +80% parried damage reduction with 3 charges used to buff up the same body part.
  • 1 Parry Buff gives +33% guaranteed parry chance and does not stack with natural base parry chance from Dexterity. If base parry chance from Dexterity is higher than the 33% from one Parry Buff applied, the natural parry chance (from Dex) value is used instead but that 1 Parry Buff still increases parry damage mitigation by 20% for a total of base 20% damage mitigation + 20% Parry Buff damage mitigation = 40% parried damage mitigation.
  • You freely place Dodge Buffs and Parry Buffs in your three areas of body (left, middle, right) by using Left/Middle/Right Stabs or Slashes. If you place 3 Dodge Buffs in left side (by using Left Stab three times in succession), you will dodge enemy's left side attacks with 99% success rate. If you then use Middle Slash three times in succession, you will have three stacks of Middle Parry Buff, giving you 99% chance to parry 80% of enemy damage aimed at middle of your body. Since, in this example, you did not spend any charges to fortify your right side, you will have base % chance to Parry 20% damage aimed at right side of your body and very low chance (when fighting bosses) to evade attack aimed at your right side.
  • To illustrate a different strategy of spending Dodge and Parry buffs, you could choose to spend your charges to buff your defences as follows: two Left Dodge Buffs, one Middle Dodge Buff + two Middle Parry Buffs, one Right Parry Buff. This gives you 66% dodge Left + natural parry %chance of 20% left damage, 33% chance to dodge Middle+66% chance to parry 60% of middle damage and 33% chance to parry 40% of right damage.
  • Only three Dodge Buff and Parry Buff stacks can be present at any given time. They can be applied to the same body side, or spread on all three body sides. Applying fourth stack of the Dodge Buff or Parry Buff removes one stack of the buff that was gained. The oldest stack gets removed.
  • The first time you apply a Dodge Buff or Parry Buff to a body side, it gets an expiry timer of 20 seconds. Adding further stacks of the same buff (up to three) does not reset or extend the expiry timer.
  • Once Dodge Buff or Parry buff timer reaches zero, the buff is completely removed from that body part. This lets you to choose whether to keep one body side fully protected with three stacks for whole 20 seconds and use that time to spam stronger attacks or keep shifting the buff around between body sides to avoid having it expire and leave you unprotected until you build up the stacks again.
  • Converting defence into offence: The Dodge and Parry Buff stacks could be expended to apply an offensive effect, for example powerful attack with potency depending on the amount of charges spent, or Double Attack Chance for next 5/10/15 seconds or 5/10/20% Haste for next 15 seconds etc. This is done so that tank can deal extra damage when someone else is main-tanking and you're fulfilling offtank DPS role.
  • As a panic button, the tank could utilise something like Third Eye from FFXI Samurai (shadows absorbing several attacks), or Dread Spikes from FFXI Dark Knight (damage shield of up to 50% of HP and returning portion of damage to attackers) or a Doppelgänger ability (if you get killed within 15-20s of using the ability, it is your shadow that got killed and you continue fight with 50% HP) or Borrowed Time ability (you take snapshot of your HP/MP/TP and buffs/debuffs/recasts and 15-20 seconds later, or earlier if receiving killing blow, you are returned to the exact same condition from when the ability was activated).

This idea bases roughly on how Puppetmaster's Automaton from FFXI was being buffed up, and also Rune Fencer and Dancer mechanics. Also many jobs from several other games utilise charge/stacking buff mechanics.
I realise the ideas above are flawed in one way or another, but I hope I gave you a picture of how this could be made to work.

And now promised random soccer video I had in mind when writing up the active dodge/parry tank idea (notice how sometimes the goalkeeper decides to jump left or right and either gets lucky, almost lucky or totally fails):
https://www.youtube.com/watch?v=dNdMDviLdr4