For example being able to edit facial features and body types like the one in EVE Online. This would essentially change every player character and racial NPC in the game. Level of detail with character model updates are one thing, but adding cosmetic attributes (eye angle, cheekbone slope, RGB color values, etc.) that did not exist before is a whole other animal.
I think it would be screwing with the game's core code and it probably wouldn't be worth it and would most likely need a separate sequel to implement.