I'm sure someone has suggested this before, but I think it would be a really cool idea to have a sort of glamour system for spells and abilities.
What I mean by this is, having the ability to change the look of some spells and abilities, while keeping the current effects and potency. I.E. swapping the appearance of Impact for Scatter on Red Mage, while still getting the upgraded damage.
Another option could be to swap out ability animations for old/removed animations, like the beloved Dark Knight damage combo, as an alternate set of animations for the current combo.
Of course, I don't know how intensive this would be on the devs end, and it's not something that would be viable in PVP, since clarity is paramount there. But I figure, if its possible to implement, it could be a great addition to the customization potential in the game, allowing players some more ways to differentiate their characters from one another.