From D&D Wiki
Idea for addition to rule
When a player takes Massive Damage SRD: Loss of Hit Points (50 points of damage or more that doesn't kill you outright), they must make a Fortitude Save of DC 15, or risk dying immediately. This could be suggested that they were torn in half or decapitated; Something 100% lethal.
What if an addition is added to this rule:
When the player (Or enemy) takes 50 points or more of Bludgeoning damage that doesn't kill you outright, the victim must make a DC 15 Fortitude Save. If the save is failed, they are rendered Unconscious.
In this way, blunt damage has less likely of a chance to kill you or the enemy and so you are allowed to survive. However, as a penalty by becoming unconscious you are considered Helpless, and therefore Prone, which ultimately makes the victim susceptible to to a Coup De Gras (SRD: Helpless Defenders).
Waking up can come some factor of a Will Save, as being unconscious is a mental state. Your brain has received trauma of some sort and has blacked out and shut down.
Perhaps a Will Save of DC 15 is required each turn. If the victim does not succeed their Will Save, they remain unconscious. To prevent comatose, the effects could naturally wear off after a certain amount of time. Perhaps this time can be a combination of the Wisdom Modifier and a roll.
Wisdom Modifier would be applied because that is what is used under the Will Save. An unconscious character may remain unconscious for something such as 3d6 turns + 1 for every 5 points over 50, including +1 for the value of 50.
3d6 + 1 for taking 50 damage.
3d6 + 2 for taking 55 damage.
3d6 + 3 for taking 60 damage.
3d6 + 4 for taking 65 damage.
This continues as such. Your Wisdom Modifier is subtracted from the total, making the full algorithm:
(3d6 + #) - WM
Suggest you have a Wisdom Modifier of +4, and you've taken 55 bludgeoning damage. Your roll for this would than be:
(3d6 + 2) - 4.
Suggest your three rolls of the d6 are: 2, 5, 4.
You would be unconscious for:
[(2 + 5 + 4) + 2] - 4 turns
[11 + 2] - 4 turns
13 - 4 turns
Unless, of course, you succeed the Will Save DC 15 in the meantime.
Obviously this is a rough skeleton, but I feel it's plausible. Anyone have any input?