Alternatively, you can just sacrifice hit die acting that they are maxed value. So, if you're a 4th level barbarian, and take 24 points of damage you could just take 2 HD worth of damage and call it good or roll any number of your HD hoping to roll higher than 24." and thinking about how this would change combat to being much more active is how I started on this idea.
Let's say Hit Points are supposed to reflect stamina instead of health, and Hit Dice are the real deal for determining whether a character is healthy. Low-level characters? Still easy to kill. But I'm getting ahead of myself. Ignore the magic system of D&D for a moment and let's just focus on combat. Hit points are a measure of stamina, and when they run out you're exhausted, can barely move, are incapable of taking action, and can only barely defend yourself from being attacked.
Hit points are still there, but when you attempt to block an attack your armor (and shield and weapon altogether) provide a defense dice pool against the damage you take. You roll the dice of your armor and if the damage exceeds it, you take the difference against your Hit Points. You can add Hit Dice to this defense, but if the damage exceeds what you roll you lose the Hit Dice instead of the Hit Points.
Shields add 1d4, 1d6, or 1d8 depending.
Armor ranges from 1d4 to 1d12.
Weapons add small bonuses, anywhere from +1 to +5.
Meaning, a low-level enemy like an orc is at a disadvantage against a well-armored opponent, but the breath weapon of a dragon is going to damage its victim regardless of how well-armored they are.
If your Hit Points are reduced to zero, you can only wager Hit Dice and one piece of armor/weapon to avoid damage.
If your Hit Dice are reduced to zero, you are unconscious or soon to be dead.
Every character has a dodge modifier that starts at +10 and is only decreased by what sort of armor they are using and the weapon they are carrying.
Most weapons are -1, but a few are -2 or -3. All polearms are -4.
Armor ranges from -1 to -5.
Shields are -1 or -2.
Meaning, yeah, you could have a -1 Dodge if you're wearing full plate and carrying a heavy shield with a halberd.
You roll a d10 and add the modifier to your roll, if it exceeds the attack roll then you successfully dodge. You can't dodge forever though because it costs Hit Points every time you use it.
What about attacking?
It changes a lot. Though I think this sort of modification would work better with Lamentations ruleset, to fit it in other systems you'd need to get rid of class-based attack bonuses. Weapons would have to add modifiers to attack rolls, or you could have weapons give a variable bonus similar to the way Dungeon Crawl Classics gives warriors an attack bonus with a dice roll.
I'm not sure what the best way to modify it would be, but having an active defense is certainly more appealing to me.
I have never seen a ttrpg create an elegant way of using parrying effectively. I'm all ears if you have an idea how to apply it in this system. For now, I expect making an attack roll against the attacker's roll is the only way to do it. Leaves a lot down to luck instead of skill, which I don't like.