This should be very simple to implement code-wise, as it would be nothing more than adding a few entries into the effects.json that get applied whenever you get a particularly nasty hit (similar to how bites from zombies can sometimes apply the “infected” effect to a limb)
Recently, in another thread, I contemplated the current health and first aid system.
Really, the idea of healing in Cata is kinda overblown, at least in my opinion. This is kind of the reason why I always take the traits Flimsy and Slow Healer. When I get myself smashed half to death by a zombie brute, I shouldn't be able to just sleep on it and be fine the next morning. I really, really hate Fallout 3 style healing like this. At least with SlowHealer+Fragile/Flimsy, it takes me several days to fully heal the damage done, like it more realistically would. Getting mauled almost to death should at the very least put a character out of commission for a couple days, maybe even a week or more, while they stay in a safe place to heal up and recover. I mean, shit, when I have a nasty fall or pull a muscle, I can feel it for a week or more! It doesn't just go away after a single night's rest. Getting damaged should be something you try to actively avoid, not just a minor nuisance that can go away by going to sleep or just using a bandage. All in all, damage means nothing because healing just happens too fast / is too readily available, and I really, really hope this gets changed some time soon.
Now, while I was thinking of what could be done to fix some of these problems, I came across an idea: What about effects that are meant to simulate injuries that don’t just go away after a bit of rest or a first aid kit?
What if, for instance, if your head gets hit for more than a certain amount of bashing damage, you get the “Concussion” effect; -4 intelligence, -4 dexterity, -2 perception, -25% melee roll? Such a thing would probably last between a few hours and a day or two. Why not add other effects of this nature? Getting hit with enough bashing damage to a body part could add a “Bruised” effect on that body part, that adds a constant amount of pain and maybe a minor (-1) penalty to dexterity? A nasty bruise would probably last for a couple days, as long as a common cold or flu. It would obviously stack too, since you can have multiple bruises to simulate varying degrees of how badly you’ve been fucked up. Or, what of over-exertion, when the player has depleted their stamina bar too much for too long? -3 strength, penalty to stamina recovery, constant pain and muscle aches, duration increases the more you exert yourself, etc. Currently depleting your stamina only causes you to run/walk slower and also gain the winded effect (but which only lasts for like one single turn so its basically nonexistent)
There are many, many different effects that can be added that go along these lines. Hairline fractures, injuring your knees/feet from a fall and making it difficult to walk, cuts and scrapes, dislocating a shoulder, breaking a rib, the list goes on and on. The point is that, currently speaking, the only things that matter are hit-points and pain, which are both very vague and nebulous - along with suffering from the problems I detailed in the quote above.