I thought the idea that when we hit max level we begin to unlock points for a sphere grid (FFX), License Board (FFXII), or what have you was cool- this merit system allowing you to spend your otherwise lost exp (because you're at cap) on cosmetics associated with that job. So as you exampled you could change/upgrade your dark knight attack animation.

Another example being the dark knight aura effect we saw in the HW job trailer that we don't actually have in game, where he burns/emits void energy. White mage may be able to break glamour rules by purchasing a holy breast plate glamour. Sort of want to make it a whole job now but also thought Golbez/Ex-Death inspired armor for Black Mage would be fun. Beyond unique opportunities to break the glamour and animations (normally not an option in game) there could of course just be rewards almost like the achievement reward system we have, and maybe even allow certain categories to have cross over points. For example you might have a dragoon with a lot of points (perhaps we can bring back the idea of anima currency lol), they have a lot of anima, and they can spend it on some jump changes (cosmetic), new mount (dragoon related), crimson gear, or an expensive glamour set that is shared between all melee damage dealing jobs. Being cosmetic only will prevent someone from reaching cap and then being told not only do they need a higher ilevel but they also need to grind 50 hours more exp for enough anima to buy the Legend of Dragoon armor and penta-thrust heavy thrust animation change.

Also could probably stick some scholar/summoner fairy/egi glamour options in that :3.