I think the concept of a Water Mage have enough potential to be revisited one more time.
Not as a skin like Atlantean Syndra.
But rather, someone more offensive and less supporty like Nami is.
For one thing, water effects are so preeeeetty. Secondly, there's just so much you can do with a compound that is commonly associated with how easy it is to find in all 3 states.
Imagine a water manipulator whose kit involves using a set amount of water and performing gaseous, liquid, or solid ice attacks. Or use water to somehow drown an opponent in the Summoner's Rift.