For a little visual flair, and potentially some practical uses, i suggest adding the ability to customise the shapes of hotbar icons and the artwork for them.

If someone just wanted their HUD to look a little more unique, they could turn their icons into circles.

On the other hand, if someone wanted to differentiate, for example, instant cast abilities from non-instant abilities, they could use different shapes for different types of spells. That's just a very broad example though.