Health. Not combat hitpoints, but general health. Maladies and malaise, long-term injuries and disabilities, fitness or lack thereof. Mortality isn’t just “the human body is a car that breaks down over time”. Bodies get better/worse, change, adapt. They get sick on foreign food, tired when awake too long, emotional when distressed.
Thematic games are starting to capture it, but holy cow, it’s usually just treated as a point buy and ignored.

All that being said, I once played a Comanche guide who was shot in a gunfight, and spent two gaming sessions laid up in a makeshift hospital in a church. Realistic? maybe. Fun, not really.