Decapitation is not unique to the head. Dismemberment is a proper catch for this game system.
Immortals cannot regrow hands, they CAN attach them. Something done not in combat, linked to their Mythic Regeneration. Which they have. not should have, have.
Living dead with, say, condition to be burned to nothing, can attach a head back and sow it or glue it
Creatures like a plant monster, or my favorite, a troll. Regrow the head or the body, or both O_o
You should take dismemberment as a catch because decapitation usually means being taken out. No head to see, smell, hear makes you useless.
Minor consequence: Cut fingers, I can't hear anything, Who turned out the lights?
Moderate consequence: Severed hand, Severed foot, There is a piece of my ass missing.
Severe consequence: Severed arm, severed leg. Half of my head is over there.
Extreme consequence: Armless, Legless, "Half of me is dancing, the other is singing", headless - should imply that you are somehow alive.