Armors, weapon skins, voices, nameplates… everything is frankly just a texture or model.
Maybe cosmetics can now have some animation. Not like animated moving picture, but a glowing consecutive beam on a nameplate or lights on armor pieces even skins.
Kind of like the confetti or flame on a helmet or character voices returning as well like back in Halo reach? These types of cosmetics will be legendary tier and actually something satisfying to achieve gaining!