A customizable survival mode allowing players to tweak their experience to their preferences would add a large amount of replay ability to the game.
Inventory settings: Settings for how players store items on themselves or in containers. Ranging from casual like TWDS&S with easy to manage slots to place items directly into with no regard for the weight of each item, to having more hardcore inventory management where you have to manage the backpack space and weight, put individual pouches on your character for carrying ammo, and attach every weapon to the player in some way regardless of if it’s a sling, holster, sheath, or some other way to attach the weapon.
Healing: healing in much the same way as the inventory would need different levels of realism, with things ranging from being able to heal from any wound without treatment but being able to speed up the process with basic healing items and potentially even food items, to having to manually stitch up, clean, and bandage wounds, potentially even having to set broken bones back in place to heal wounds, or being able to lose limbs entirely to permanently lose functionality until dying and respawning on the furthest end of realism.