Q- Water form: basic attacks gain range and do magic damage. Press Q again when in this form to preform Water Whip: Lash at an enemy with quick strike of water, dealing magic damage.
W- Earth form: Basic attacks are slower but do more physical damage. Press W again for Metal Bending Spear: Use metal bending to create a spear and lodge it into the enemy, dealing physical damage.
E- Fire form: Bend fire blades in both hands allowing for a slight basic attack range buff. Basic attacks also have a slight chance to burn a target for a few seconds. Press E again to preform Lightning Bending: Use lightning bending to strike a target with lightning dealing physical damage and having a slight chance to paralyze the target for a second.
R- Air form- With elegant steps as light as air gain a movement speed and attack speed buff (No change in basic attack range this time) Press R again to preform Glide: Use air bending and a staff to soar high into the air for a few seconds, massively increasing speed and making you untargetable (however you also cannot attack) Press R again to decast or the ability will end after a few seconds and prevent further casting of bendings forma few seseconds
I have not thought of a passive nor have I thought about balancing issues, I just thought of this in a few minutes but it sounds like it could be neat to me.