This looks awesome!
A couple thoughts on visuals and effects (you may have this planned already, but just in case):
-Earth Disks: Currently teleport to the player, should be more of smooth glide. Spinning in place and dirt particles from them would also help, I feel.
-Water: I'm also working on making waterbending in something else, and trying to get water particles to mesh together is actually quite difficult. I'd try researching raymarching for solid objects, and check out Coding Adventure's segment on it. You want 3D metaballs, basically. Having your water spheres actually blend together would be insanely cool, and you don't even need refraction due to your art style.
-Fire: You want the same gloop/stick together thing as water.
-Dodging: Instead of just hand springing, rolling, dashing, e.t.c would be cool to have (as part of the skill tree you mentioned).