...reducing the max stamina, health, and mana - H/M/S fatigue - as the character grows more hurt and tired...
I think that would be a really good way to handle it. I started planning out a system a while ago for tracking this sort of thing, kind of a more intricate version of the TES stamina system. Every activity in the game would be assigned a power output, based more or less on real world numbers (watts I guess.) Running would consume so many watts, as would swinging a sword or drawing a bow, or jumping. There would then be two energy bars, one for long term stamina and the other for short term power (relating to slow and fast twitch muscle fibers, respectively.) An activity would draw from one or the other of these bars depending on how strenuous it is. The stamina bar would take a long time to deplete (like an hour) but would also take a very long time to regenerate, possibly requiring sleep to get all the way full again. The power bar would both deplete and regenerate much more quickly, on the order of a minute or less. As your character became more fit, the width of the bars would increase, allowing your character to perform actions more times before getting tired. But the threshold values used to distinguish between stamina and power activities would also increase, so eventually "power" actions like swinging a warhammer might become stamina actions that could be performed effectively indefinitely.
For something like running and sprinting, your run speed would be determined by whatever power output would keep you within the stamina threshold. Sprint speed would be determined by your power threshold.
For weapons, basic attack speed might be effected by your stamina threshold, while power attacks would become more powerful as your power threshold increases. Some weapons might just be entirely unwieldable for weaker characters.