"Stagger" as a nuance-capable party survival tool.
Since 1.x I'd been pushing for a revision of the XIII Stagger mechanic. Consider all damage as the product of Potency and Pierce. In XIII terms, this is the raw, initially Ravager-like effect of Stagger and the factor by which it can be modified towards the longer-lasting Commando-like effect that keeps it from draining as quickly. Jobs are balanced around having either more Potency or more Pierce by which to put out their intended damage levels. Those with the most Pierce, such as MCH, BRD, and DRG, are good at longer-term harassment of enemies through Stagger. Those with the least -- and therefore the most potency to compensate -- are good at immediate, short-term Stagger. But, this doesn't lead to an incapacitated effect. Instead, it simply directly saps from a combination of the enemy's Potency (based on the Potency of your attacks) being generated or readied at that time, draining over time and upon the drained skill's use, and Strength/Dexterity/Vitality (based on Pierce damage). In this system, Attack Power is integral to means of defense -- without it, you cannot significantly block or parry.
In this system, active mitigation would also be based on readied potency and upon one's main stats. Being bashed about reduces the strength with which you can block or parry right now. Being gutted reduces the main stats by which you can increase block/parry strength, block/parry chance, and your maximum HP and HP restoration over time and from external sources. Being overwhelmed will kill you. The advantage of tanks, then, is less that they can survive a single blow or flurry of blows that otherwise would not be able to, but they can continue to take hits without being trampled, and with the least negative impact on their own damage (proportionately, and perhaps even flatly). This allows non-tanks additional lenience by which to participate in tanking tasks, albeit only momentarily, and reason to do so. Inversely, the tank's survival depends largely upon the party focusing down and/or suppressing enemies who would do the most harm so that they tank can survive. Survival is now everyone's concern.
The key here as it applies to the present state of tanking -- party irrelevant -- is that not all skills Pierce equally. Enmity skills, therefore, would be replaced with skills highly capable of immediate Stagger, while everything else is invested with utility that allows you to tap into your resources, ready yourself, ramp higher, or whatever else possible in the time that you needn't bash an enemy back just before their blow so that you can survive by bare hundreds of HP. For Warrior, Skull Sunder has the most stagger immediately producible. Butcher's Block has the most over three hits, sustaining the Stagger but spiking for less. Storm's Eye peaks at the end of its animation and adds Armor penetration. Storm's Path sustains, heals, and has twice the gauge benefit of any other skill. So on and so forth.
Debuffs and DR.
Replace Diminishing Returns. Replace most pure Stuns/Silences. In their place, create spectrums of debuffs, with multiple tiers therein, inflicted with debuff "damage", so to speak. Stun degrades to Pacification/Silence which degrades to Slow (Action Speed). Bind degrades to Snare* which degrades to Heavy (Movement Speed). (Tugs, slowing heavily rather than completely, like trying to pull free of a trap, taking effect over displacement rather than breaking instantly upon movement again becoming possible.) Essentially, players universally and mobs by rank, type, or uniquely are given initial and end thresholds (think, static vs. kinetic friction coefficients) for a (de)buff of a given spectrum, be it Action (Haste <> Stun/Pacify/Maim), Movement (Alacrity <> Bind/Snare/Heavy, Vertical (Launch <> Smite), Incapacitation (Sleep/Apathy/Daze), or whatever, for when the spectrum will allow the next tier of effect. If a player fails to deal enough Action Speed-type debuff "damage" to meet the start threshold for Stun, it'll be degraded to whatever tier it can afford.
This debuff "damage" works on literally anything, but, naturally, at less effect the more mass or HP or resistance or whatnot an enemy has. Moreover, each time a debuff of a given debuff "Damage" amount is inflicted, resistance is added proportionate to that "Damage" over a second or so, with drain speed being inversely proportionate to the %effectiveness of the debuff that created it. A Stun has 100% effectiveness in preventing actions and Bind in preventing movement, so their resistances do not decrease while Bind is active, giving it a longer period during which some portion of Stun or Bind "Damage" is reduced. This means that debuffs, e.g. Stuns, are hard to stack continuously, because the more -say- Stun you hit for, each following stun is worth cumulatively less until the resistance has had time to wear off. This is your DR component, but done flexibly.
Let's say one player tries to stun a mob with some 1500 Stun "damage", but its threshold is 2000, then it instead inflicts Pacification and drains continuously based on the damage generation (e.g. building continuously towards the next special or two coming off cooldown but locked out due to pacification) being prevented. Let's say the mob builds 20% resistance, so 300 debuff damage mitigation. If another ally a second later hits the mob with another 1200 Stun damage after 200 has already drained and 280 is reduced by resistance (20 faded over the second), then we're looking at a total now of 2200, enough to inflict the full Stun. The Pacification is then consumed instead as a Stun, and the debuff Action Speed debuff "damage" is now consumed at the higher drain rate of a Stun, during which resistance does not fade.
Again, just as this works for players against mobs, this also works for mobs against players. Tanks will have higher thresholds, drain rates, and inflicted-resistance modifiers inherently atop just taking less debuff damage in the same way they take less HP damage.