Does it sometimes feel unsatisfying that a fifth-edition character can drop to zero hit points (essentially be on death's door), receive healing, then be back to full fighting strength with absolutely zero repercussions? If so, this lingering injury system might help.
Basically, it presents a very simple set of rules whereby there are mechanical consequences--lingering injuries--when a character drops to zero hit points. These lingering injuries are designed to introduce consequences to the game, but NOT heavy-handed ones.
This ruleset is designed for fifth edition, but can be adapted for others with a little effort.