Its a common bit of knowledge among army medics that pain can literally kill a person by causing circulatory shock, hence why morphine is so important in determining survival of wounder soldiers. Not only that, but as many might be able to attest, experiencing a great enough pain can actually knock you unconscious. This should be implemented in-game, really - as currently the only thing pain does is reduce stats and speed, which I think is really downplaying how crippling intense pain can really be. Pain should be something that can actually pose a threat towards the player’s mortality, not just a stat debuff like going through amphetamine withdrawal.
For instance, stepping on a landmine with little to no armor should realistically cause enough pain to knock you unconscious, at which point you would bleed to death or be killed by a roving monster. Characters really shouldn’t be able to survive things like that unless they’re wearing really good armor made from kevlar or above. Similar also to being shot with bullets like from turrets - many times when people are shot they simply pass out from the shock and pain of it, hence why most people just crumple to the ground after a single bullet wound, even if it didn’t hit anything major like the heart or brain.
Also, adrenaline spikes should temporarily set pain to 0. On top of that, adrenaline should be triggered by more things, not just being near-death. Like, really loud noises, seeing a huge swarm of enemies approaching, being shot at, being near an explosion, etc.
Every person experiences an Adrenaline rush if they are in a dangerous or combat-oriented situation ; therefore it would make sense for every person to have an adrenaline rush, in various situations - especially at the start of the game, where the Soon-to-be Survivor (probably) never met a Zombie - or giant monsters and wild, dangerous animals. I’m not sure how easy you could create such an adrenaline system though - it might work best if every monster (and npcs for that matter too) have a fixed ‘Danger’ Value ingame, and the Player has a danger-limit, a threshold. If the visible monsters/situation is above said Threshold, an adrenaline rush starts, improving your stats, reducing pain and improving stamina. Said Threshold would slowly rise, the longer the player survives, up to an upper limit, which would be the ‘finished’, battle-hardened Survivor, who fought hordes, jabberwocks, visited labs and so on.
This Threshold could also be used for stuff like ‘too scared to move’ - if the situation is double or triple your danger-limit you could be scared shitless, thinking ‘im dead’ and all that stuff, and simply not move for a round or two, before the survival instinct kicks back in. That would only be human. In return, a high danger-threshold could prove to be a light mental immunity against horrors and such.
The Adrenaline trait on the other hand could either play with the adrenaline threshold - or it could improve the boni you get from an Adrenaline rush.
However, as I said before - I have the feeling that this system would be very, very hard to implement, and it would take away from the roleplay experience and might be frustrating if your player stares blankly at 2-3 zombies for an hour, causing him to die.