As I tend to start with, several assumptions:
- Most damage is small numbers, rarely higher than 6, very rarely double digits.
- In accordance, the damage you can soak is not a lot, but it can be relatively exceptional.
- High damage is indicative of a terrible foe, one who can kill you with considerable ease, if they just put their minds to it. Fighting something with a high damage is not something done directly.
- Armor assumes this concept. The heavier you go, you trade your long term mobility and relative efficiency for short term survivability against harsh combat conditions.
Combat Fatigue or Avoiding Deadly Blows
To my knowledge, following some research, combat is more about avoiding any blow, as any blow can prove to be fatal. Even more so when weapons are involved. To go with that, the majority that any character in combat is doing is to avoid deadly blow - by way of dodging or armoring up. Whichever it is, it means that they are tiring themselves out as they do it. Dodging is extraneous and armor is heavy.
Taking inspiration from Fate, mostly, I envision a grid 5 cells wide and several lines long. This grid acts similarly to stress in Fate but assumes a somewhat greater level of potential hardiness. Characters would start with one or two lines. When they take damage, it first goes into here, filling into no more than a single line. If the line fills out completely, you have avoided the blow entirely. If it did not, it spills into-
Hit Points or Your Own Token Variation
If something hit hard enough to take any chunk of hit points away, things are not good. Taking actual damage should leave scarring and is going to take a while to bounce back from. This damage, even small, would indicate an actual wound, as opposed to the cuts, scrapes and bruises that are included in combat fatigue.
Any armor would do, really, to avoid terrible fates. Even padded clothing could save you from some blunt trauma. In general, going up to having a chain shirt would be within the definition of wearing light enough armor to not really feel it. Glossing over how realistically a chain shirt would be defensive, you get what could be the light side of the medium class of armors in many games. The rule is that the heavier your armor, the more protective you can expect it to be.
But once you hit heavy armor, you are met with a ticking clock - the armor, by virtue of being really quite heavy, will be chipping away at your combat fatigue. When going about with such a heavy armor, your options end up being haste between fights, several fights in a row or well earned rests whenever possible. Whichever option you choose, it won't make adventuring any easier. In some cases, heavy armor may even restrict certain activities which might require finesse or manual dexterity.
This system would favor athletic characters - whichever way is chosen to reflect that. Fate does provide more Physical Stress for higher levels of the Physique skill. D&D models this by giving the fighting classes better hit dice. Mind that this system is not suitable as a replacement in most cases, since it would take a lot of tweaking on its end or on the other end.
Whenever the milestone for more combat fatigue is hit, another line should be made available for the character. In other cases, maybe another cell on each line might open up. The idea is that more combat fatigue unlocking could be another blow deflected, maybe even two. However it is done, it will probably save the character's life in combat.
The system makes several assumptions which I don't recall finding in sufficient force in games I know. Some assumptions, on their own might exist, but not in this particular mix.
And this system is still just an idea put on paper, so I'm not quite there yet with what to do with it.