In the armor revamps I've played around with in my head, the first half of your health is your 'stamina', with any damage received after you reach half health (when you're 'bloodied' as it were) being more serious damage. Stamina you can get back more easily, but damage received after that would need active healing to regain. Critical hits bypass your stamina, and sneak attack bypass your armors damage reduction, and possibly your stamina too.
What I've never figured out is how to decide what AC is after armor becomes just a source of damage reduction. In my head being a Dex fighter should become less and less effective during a fight, as your acrobatics and fencing take a toll, but this is possibly no longer heroic fantasy, but without a dedicated defense skill it does seem like Dexterity and possibly Strength are what we have left.
The Starfinder system has a mechanic similar to this. Essentially you get back all your Stamina through 10 min rests, but your HP is only naturally restored by ~[level + Con mod] each long rest. To get more HP recovery, you need to use spells or spend days resting with medical care.
I've seen an armor system (maybe suggested online, maybe in some official system, idk) where armor had both AC and DR. The difference between the attacker's roll and your AC was added to damage. E.g., 23 to hit AC 17 -> you deal normal weapon damage + 6. And then DR was applied to that damage total. It makes glancing blows (17 to hit) deal minimal damage and high rolls can be pretty meaty, theoretically making combat more descriptive and varied in terms of the effectiveness of each strike.
To be clear, I don't particularly want such an above system. I'm perfectly fine with 5e's all-or-nothing AC system. Although I would like (subtractive) DR to return in some form in D&D - the immune/resistant/vulnerable system is too broad imo.
