When you are DM, what sort of variant rules do you like to give to your players? Not necessarily game-changing rules, but rules to give the game more flavor, or to try and encourage better roleplaying. I'll give a few examples first.
DEATH AND RESURRECTION:
Coming back from the dead is not perfect. When a character is revived, even with a wish or miracle spell, they bear the scars that took their life from them. The decapitated Dwarf barbarian has a jagged scar around his neck where the ax took it off, the Elf wizard has a circular wound on his front and back where the lance went through him, the human paladin's body is covered in pock-marks from the necrotic disease he was cursed with while fighting the lich, the half-orc fighter has a noticeable rope-scar from the time the party was unable to convince the village that he wasn't part of the recent raid in time. These scars are noticeable different from similar scars that were recovered from (anyone can somehow tell the difference between the Paladin's pockmarks and those of someone who recovered from the same disease, and the marks around the fighters neck are just different enough from the depressed merchant who tried to hang himself but was cut down and recovered.) This provides no mechanic difference, but people may react differently. The barbarian clan now ostracizes the Dwarf because he gave up an eternity in the fields of honor, guards are more suspicious of the fighter because he looks like he tried to escape punishment by resurrection, peasants may hate the characters because they were rich enough to cheat death. Alternatively the Paladin's church may exalt him for returning to the fight after death. It depends on the nation's views. This helps to discourage some degree of recklessness, especially in high level players "Does it matter if this kills me? The cleric can cast resurrection 3 times per day!"