Removing Hit Points
The fort save is probably already familiar to most, but here I present a different take on it. I have another radically different system to present later. The system below is a rough draft. Edit: I'm out of time, so I'll to come back and fill the gaps later. Let mo know what you think so far.
simple alternative is to use hitdice plus con modifier instead of the fort save, in what is called a soak roll.
Every time you take damage you gain a wound point (after the current soak roll results are resolved), roll your hitdice and add your con modifier and subtract wound points. If your soak roll is equal to or higher than the damage taken, you only gain the wound point.
When you fail a soak roll, in addition to the wound point, you also gain an Injury condition (detailed below), but if you fail by 5 or more, you also gain the Dying condition (but may possibly still be concious, make a will save DC10 to remain concious), and if you fail by 10 or more, you are automatically unconcious and additionally must immediately make a fort save or die (DC 15+wound points).
Nonlethal damage
Non-lethal damage gains wound points normally, the difference is entirely in the soak roll and conditions gained from failing it, thus making a soak roll vs lethal damage ignores whether wound points came from lethal or non-lethal damage.
If a soak roll vs non-lethal damage fails, a character is dazed for a round. If they fail by 5 or more, they becomed Staggered. If they fail by 10 or more, then they are also unconcious, but still Stable.
Additional non-lethal damage accrues wound points normally, but failures of the soak roll treat it as lethal damage but with a +5 to the roll.
Healing
For an uninterupted 8 hours of rest, a character loses 1d4 wound points per constitution bonus, minimum 1, and loses one Injury. Using the Heal skill to provide care doubles this rate of recovery.
Cure spells recover wound points exactly like hit points normally would, however, a cure spell also heals one Injury per die that rolls 4 or higher.
Restoration spells heal a number of Injuries equal to spell level and may also be spontaneously cast by a character that is given a special ability to spontaneously cast cure spells (I.E. a cleric can, but not a Bard).
Fast healing works as normal but on wound points.
Regeneration works on wound points. Once all wound points are removed, then injuries start being removed, taking as much time per injury as it takes to recover 15 wound points through this ability.
Extra Hit Points
Additional hit points such as from the Toughness feat or temporary hit points from a spell or effect instead are bonuses to the soak roll. Extra hit points from a high consititution are not added since constitution is already added to soak rolls.
Damage reduction/Resistances
These reduce incoming damage as normal. If the damage is reduced to zero or less, then no wound point is gained.
Damaging Objects
When damaging an object, treat it's hardness as DR, and it's hit points as the soak roll result. For substances with hit points-per-inch, taking that much damage chips the material one inch deep and damage over that amount goes into the next inch to potentially deepen the chipped area. Gaining wound points in this case implies dents, scratches, etc, a general degradation of the item. Injuries gained are actual breakage of the item. If the item would become Disabled or Dying the object is Broken instead. If the object would become Dead, it can't function in any capacity, and repairing it takes a adds +5 to the DC.
Creatures with no constitution scores
Makes no difference, they simply lack a bonus or penalty from constitution.
Coup de Grace
Assuming a character can discern a vital location (make a knowledge check for unfamiliar creature types that lack the normal obvious spots such as carotid arteries), and roll a straight d20, if the result is higher than their Constitution score, than they die, if the result is higher than their constitution modifier they are Dying and Disabled. If the result is less than their Constitution bonus, then they are alive but gain an Injury.
There is no Massive Damage Threshold.
New and altered conditions,
Injured
An Injured character has Injuries. For each injury reduces their speed by 5'. The character also takes a penalty to physical ability scores and saves equal to the number of Injuries they have. If your constitution score drops to 0 or less due to having Injuries, you gain the Dying condition.
Dead
A dead character is unconcious and all sustained feats, abilities, spells cast by the dead character, etc cease to function. If a dead body loses an injury or receives resusitative healing (a heal check vs DC 20 + the dead character's Injuries) and the character has fewer Injuries than their Constitution score, then the character can make a stabalization check, which if successful, they are no longer dead and instead are Dying. Without such healing, a dead body gains an Injury every round. Once a character has a number of injuries equal to their constitution score, the character can no longer be revived through simple magics or the heal skill and instead requires healing the body or a Raise Dead or similar spell (if such magics exist), however, Injuries can still be removed until the character has few enough to be revived. After a number of hours, the soul departs and after that point only a Raise Dead or similar spell can revive the character. The number of hours in which this occurs is determined by rolling 6d4 plus 1d4 per charisma bonus (or minus 1d4 for a charisma penalty). Any spell that works on the living also works on the dead prior to the soul's departure. Spells that turna dead body into undead don't work till the soul has left the body, unless it would normally work against the living.
Dying
A Dying character is not automatically unconcious. A Dying character is also Disabled. Each round, a Dying character makes a check to lose this condition by succeeding on a Fort save vs DC 10 or they gain another Injury and remain Dying. If a Dying character gains a number of Injuries equal to twice their Constitution score, then they become dead. If the character ceases to be Dying, they become Stable.
Disabled
This works generally as normal, except the following,
-when the character would lose a hit point such as taking a strenuous action, an injury is gained instead.
-When making a daily constitution check to start healing, a failure gains an injury instead of losing a hit point and the penalty she takes is equal to her Injuries.
-Whether a character is concious or unconcious has no effect regarding this condition save that actions may only be taken if awake (unless they could normally be taken while unconcious).
-Additionally, a character may take a full-round or longer action by increasing the time to two rounds, or doubling the time taken to perform the action (whichever is longer) to do so carefully (subject to GM discretion, some actions may be deemed too strenuous to be performed carefully at all, I.E. making a strength check to bash down a door), but this action requires the character to "drop their guard" and thus automatically allows an attack of opportunity and the character counts as flat-footed.
Stable
A character that is not Dying is Stable. A Stable character does not gain an injury every round. A stable character can be concious or unconcious, if unconcious, they can make a DC 10 Constitution check each hour to become concious. Alternatively, a perception check can be made to awaken to being slapped, splashed with cold water, etc, though a penalty is taken equal to Injuries. Smelling Salts or similar can awaken a character without a check if Injuries are less than their Constitution score. A character is normally Stable, unless Dying, thus, when unconcious from non-lethal damage, these rules still apply to waking the character.