Would be cool if level 4 spells used a different colour ^_^ kinda like what MythofSoma did
Official client did have different animations for level 4 & 5. It's how one of the characters I inherited from QGO after they shut down got banned on the Wemade servers that came up after.
When QGO client was active the spells didn't have any different animation, when the Wemade version came online it included the new animations and having level 4 defiance I looked a bit out of place casting it.
As for changing skins, with things like Magic Shield that introduces some difficulties, since if the newer spells were added in the future like Improved Magic Shield that replaces the current one and is orange in colour.