This is the setup my group will be playing with in an upcoming campaign. It's going to be potentially very... "Meat Grinder"-y.
First, we're using our typical setup for Critical Hits
Second, and more importantly, we're using the Wounds and Vigor rules
In tandem with the Wounds and Vigor rules, we're also using the Wound Threshold rules, as presented in Pathfinder Unchained (mostly).
Now, the Wounds and Vigor rules don't quite go into extreme detail beyond how they function in a bubble and with a few other rules (like healing), and don't EXACTLY cover how they interact with other systems or even common rules like Bleeding damage, so I had to figure out how those would work.
The result was an utter bloodbath, which I'm... kinda totally okay with.
SO.
WOUNDS & VIGOR WITH...
Called Shots & Massive Damage
The Massive Damage and Debilitating Blow values for Wound & Vigor are equal to the creatures’ Constitution score (no minimum). This completely replaces the usual "half it's HP, minimum 50 damage)" rule.
When a creature takes Wound damage equal to its Constitution Score in a single, non-Called Shot hit, it must succeed on a DC15 Fortitude save or immediately die.
If a creature takes Wound Damage equal to its Constitution Score as part of a single Called Shot Attack, that creature receives the Debilitating Called Shot effect of that Attack.
Critical Called Shots don't need to be changed - they activate when a Critical Hit is scored in a Called Shot, and this functions the exact same way with this system.
Saving Throws for Called Shots are already described in detail and they work perfectly fine with this system, as well (since they're based not on the Damage dealt, but on the AC hit).
Wound Thresholds
Wound Thresholds are calculated with Wounds alone, though using the Gritty Mode.
To wit:
A character with 18 Constitution has 36 Wounds. While they have 36-28 Wounds, they are Healthy and take no penalties; 27-19 Wounds, they are Grazed and suffer a -2 penalty; 18-10 Wounds, they are Wounded, they gain the Staggered condition, and they suffer a -4 Penalty; 9-1 Wounds, they are Critical, and suffer a -6 Penalty; at 0 Wounds, they are Dead.
If a Wounded/Critical creature takes a Standard or Move Action on its turn and fails its DC10 Constitution Check, it falls Unconscious and gains the Dying Condition.
Bleed Damage
Bleed damage, like Critical Hits, is automatically applied to Wounds.
Constructs
Under the Wounds and Vigor system, Constructs simply have HP/Vigor; Constructs already have no Constitution score at all, so they simply stop moving when their Vigor reaches 0 (they turn into Objects at that point).
Undead
Undead have Wounds equal to twice their Charisma score, since they replace their Constitution with Charisma under the normal rules for calculating Hit Points.
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What all this does together is create a very, very nasty game, where even non-magical, mid-level characters with non-masterwork weapons can cause the death of high-level full-casters with surprising ease.
Rather than be a minor annoyance under the normal Hit Point rules, Bleed Damage is actually extraordinarily deadly.
This makes the Wounding weapon enhancement, the Brow Gasher spell, the Bleeding Attack Rogue Talent, and the Bleeding Critical feat, among a plethora of other abilities, extremely useful.
Suddenly Rogues focusing in Bleed damage and Fighters using Critical Feats (especially when combined with the Stamina ability of Bleeding Critical) are some of the most-devastating combatants on the battlefield.
While it DOES make things like hitting Debilitating Called Shots more difficult when an enemy still has plenty of Vigor, it also makes them MUCH more likely after you've worn them down. (it also, again, makes things like Bleeding Attack or Bleeding Critical absolutely DISGUSTING in how easily it becomes to lop someone's arm off, for instance).
Oddly enough, this also, unfortunately, makes the Holy Vindicator Prestige Class very, VERY dangerous to its user, as you'll be taking a pretty substantial amount of Bleed Damage if you choose to use your Stigmata ability.
Then again, making one Prestige Class very deleterious so that you can get all this blood-&-guts goodness is something I and my playgroup are pretty okay with.
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I decided I'd put this out there for anyone who was trying to figure out how things like Wound Thresholds would work with the W&V system, as well as things like Bleeding and Called Shots, as I've seen questions concerning their interactions, but I haven't seen a consensus on what, if anything, would work.
So, there you have it - Pathfinder: Frank Miller Edition (sans the misogyny, bad dialogue, and rampant racism).