Here is a random idea...
This is similiar to Vitality/Wound, but not quite...
You have 2 "pools" of damage soaking available.
The first we will call Stamina. Your "stamina" score is basically "hit points" as we know it. It increases with level, like current hit points do.
The second score we'll call Hit Points. Which is the point you actually get hit. Your Hit Points are equal to your Con modifier. They do not go up, unless your Con modifer goes up OR if you have some sort of feat or ability that may increase your Hit Points. For example, a Fighter gets +2 to their "hit points" score.
When the DM makes an attack roll against you, and hits your Armor Class, the player has the option to decide if the attack was really a near miss or glancing blow, or if it was a real solid hit. This is determined after the DM rolls damage, so the player can decide after the fact. If the player chooses to make it really a "miss", he expends his Stamina, and the damage is taken off of this score. If he decides that it actually did hit him, then the "damage" the DM rolls doesn't matter. It is simply considered a "hit" and he takes 1 point off of his Health.
Character's do not go below 0 Stamina. There is no "negative" Stamina. Once you hit 0 Stamina, each hit is a real hit, and deals 1 hit point of damage (in most cases).
Critical Hits and Sneak Attack bypass Stamina, and deal 1 Hit Point of damage. In this case, the player doesn't get to decide, he got hit.
Also, some monsters may deal extra hit point damage. Perhaps an Ogre actaully deals 2 hit points of damage, because their attacks are so strong.
Dying
You become Staggered at 0 Stamina. So you can only take 1 action each round.
You fall unconcious when your hit points reach 0. You die when your hit points reach -Con modifier.
Healing
You recover all Stamina after an extended rest and 1 hit point after an extended rest.
Magic can heal hit points and Stamina. Hit Points heal per Hit Die rolled. So a Cure Light wounds will heal 1d8 Stamina and 1 Hit Point. Cure Moderate Wounds will heal 2d8 Stamina and 2 Hit Points. Etc.
What does this allow?
An Ogre hits your AC and deals 30 damage. You can take that 30 damage off Stamina, to dodge out of the way, or it actually DID hit you, and you take 2 hit points of damage. Perhaps 30 damage is a huge chunk, and the player is at full hit points, and he wants to risk it (hoping a critical hit doesn't come up), so he lets the attack hit him.
Also, since it is tied to Con modifier, this is generally going to be a low score. You can "realistically" only take so many "hits" before dropping. A Fighter with 16 Con would be able to take about 5 solid hits before dropping (+3 from Con and +2 from being a Fighter). A Wizard on the other hand can only survive maybe 2 or 3 hits depending on their Con modifier, so they will mostly have to rely on their Stamina (a bigger pool or resources).
Anyway, just spitballing here...
