Here's a possible idea. The basic idea is to keep the whole concept of "hit points" == "plot armor / being able to avoid getting hit", but to make this concept explicit in the system, and to generalize it in a way that allows for more "logical" effects.
Basically, here's the way it works:
1. You have "hit points" (HP) as normal, with ways of gaining them back, etc. The rate and method of gaining them back depends on what kind of system you want - harder to gain back means more emphasis on avoiding combat, etc.
2. Here's the core of the system. Each attack coming in (that didn't completely "miss") is characterized by an ordered pair (X, C), where X is a number of "hit points", and C is a condition (like that your leg is broken, or you are blinded, etc.) Now the key is that when you get hit, you can choose either to accept condition C, or to spend X hit points to cancel the effect. You can only spend hit points to cancel the effect if you have enough hit points remaining. If you cancel the effect then you have to narrate how you managed to narrowly avoid the attack. Optionally there may be other restrictions, like maybe you can't cancel it if you are unconscious.
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Of course, if C always equals "you become dead/unconscious" then this reduces to the standard hit point system. But there are other ways to use this:
1. One common criticism of some combat systems (at least I've heard it w.r.t. 4e) is that certain powers, like disarming enemies or gazes that turn people to stone, are desired for thematic reasons but are hard to implement because they can effectively bypass the hit point system to disable combatants, thus making the powers overpowered. With this system, it is possible to implement these more fairly: let's say a normal attack to kill has a lower X value but its C effect is "target is dead", while a disarm attack has a higher X value but its C effect is "target is disarmed." Then players would have an incentive to use the disarm power because it forces a choice between being disarmed and taking lots of damage, but it wouldn't just be able to neutralize the BBEG in one hit.
2. It reduces the impact of the "death spiral" because you have to get through all the enemy's hit points before you can inflict crippling conditions on them without their consent. But it allows for the possibility of injuries with interesting effects during combat, and adds strategy - do I spend HP to avoid the condition, or do I take the condition and save my HP to avoid a worse condition in the future? (IIRC, there was a thread or two in the 4e house rule section about systems where you can coluntarily take a 'wound' to get back hit points - this effectively makes similar tradeoffs a core part of the system.)
3. It could provide a unified way of representing defenses. For example armor could be HP that can only be used against physical attacks; an "amulet of magical protection" could be HP that can only be used against magic, etc. This would eliminate or reduce the need for lots of different defense mechanics like AC, different defense values, saves, spell resistance, etc.
4. It would avoid the problem of "If 'hitting' someone on the attack roll doesn't represent an actual physical 'hit', then how come my poison-tipped arrow still poisons the target on a 'hit'?" In a situation like that, the poisoning would be in the C effect, so he would only be poisoned if he didn't spend the HP to avoid it (i.e. accepted the actual physical 'hit').
5. If desired, some effects would have other "costs" that need to be paid to avoid their C effects, rather than just hit point expenditure. For example a fireball might make you move out of its AoE (to "dodge" it) in addition to paying thie hit points. So if you were immobilized you couldn't pay that cost and would have to take the C effect. More combo opportunities.
I've never thought about that in such a way before.
