So, something that bothered me a bit since the implementation of stamina was that everyone gets the same pool, excepting some mutations and bionics which increase / decrease said amount respectively.
I would suggest a stat akin to health called ‘Conditioning’ or ‘Fitness’, built up and maintained by physical activity. (Swimming, sprinting, fighting, biking, etc). Which would have a noticeable impact on stamina burn and recovery if a high enough level is maintained. (Perhaps even movespeed)
The inverse would hold true as well. With a months-long spell of sedentary base-building or car repair seeing survivors lose a bit of their edge.
Ideally however, it would be slow to build and slow to degrade sort of stat. Usain Bolt could probably have a bad week or two and still be one of the fastest men alive.