This is more for the gem shop i had this idea to offer a slot next to your skill that you can give it a cosmetic look about it say for pets give them a skin that has armor on it.
For projectile spells give people the abilty to change the way its cast like instead of 1 fireball it would be a volly of fireballs to that one target or a beem of fire.
Not sure about AoE’s but this is just a mock idea.