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A bear totem barbarian, at 3rd level with an AC of 17 would basically have 68 extra health, doubled as long as they are not fighting psychic damage.
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This would make abilities that give resistance, like Rage, much stronger, since it effectively doubles the power of armor vs. A 15 AC would then provide 30 HP bonus for a first level character, but more for a progressively more skilled character. Thinking about it a bit harder, seems like an alright area. My point is, you need to find out what equation you would use to decide how much temp HP.
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Of course, that is completely arbitrary on my part. So, the 15 AC would be replaced with the equation For example, assume a 15 AC protects a 1st level adventurer from 7 1d6(4) hits per short rest. As a result, you need to figure out a general average number for how much HP a given AC normally defends. You never roll to hit again, but instead only roll damage. On your houserule, it would need to be a substantial number of temp HP. Doesn't bother me, personally, but I can see it bugging someone. Of course, this means that a "Miss" in game terms could still be a narrative "hit" on your armor, while a "Hit" in game terms can be a Narrative miss. Stronger armors replace this skill with a solid foundation, allowing you to bear the brunt of attacks despite not actually avoiding them. Because HP can represent "luck", you can prevent losing luck by dodging on purpose, hence Dex being added to AC. It is tied to HP being ambiguous, I think.
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