A water agent would be pretty cool. They could have a river ability that pushes enemies away, and the ulti could be a very big version of this where there's a lot of water, with some radial momentum and with water physics, so that you can flush sites from heaven. The other two abilities could be a smoke (fog) and a slippery AoE that, among other things, makes aiming harder
I agree, I think there's lots of cool potential for a waterbender fantasy character on the roster, some interesting ability concepts there. Maybe some day!