My long-term plan for injury handling is to completely replace HP with a system that tracks individual wounds, from mild scratches and bruises to broken bones and large cuts, to concussions and eventually fatal injuries.
The basic progression would be that a healthy, unwounded person would have a rather large change to dodge or deflect most ordinary attacks (we’re talking random zombies flailing at you here, brutes and gunfire are another matter entirely). However this would take a toll by depleting stamina and by accumulation of minor wounds, which would cause pain and slow you down more. Once you’re impaired a bit, those attacks aren’t so easy to shrug off or dodge, and you start taking direct hits, causing more serious injuries. Obviously it goes downhill pretty fast at that point, so you’d best get away to treat those wounds.
At the same time, many of those wounds would need treatment, both to manage the pain they cause and to prevent infections. First aid would no longer heal wounds, but instead ‘treat’ them, putting them in a state where rapid healing can occur and you can usually get by with some treated wounds, which is good because I’m thinking they’d take a while to heal based on their severity and your overall health.