Spell/Weaponskill Glamours.

Some skills are just straight numbers upgrades that are done via a trait. The skill is entirely replaced on the hotbar, and never to be used again outside of level sync.
Examples:
Steel Cyclone -> Decimate
Stone IV -> Glare
Heavy Shot -> Burst Shot
Danger Zone -> Blasting Zone

Some people prefer the animations of old skills to the new skills, or might want to change things up for variety's sake. Even if it's only client side, I feel that it would still be a nice addition.
Some skills would be have to be exempt, of course. If the new skill has a gameplay effect that is more than numbers or cosmetic (for instance, the animation lock on Jump -> High Jump), then you wouldn't be able to glamour it.

Thanks for reading!