It's kind of hard to have an incredibly varied and versatile powerset in a video game, simply becuase you have a limited set of inputs. So you would normally have a small set of powers that each serve a purpose. But then doing that and still representing 4 elements means each only gets very limited options.

Thinking about it, I can see two ways to make bending feel powerful, versatile and give a good representation to all elements. 1) maybe the best solution would be to have customizable load outs with various bending powers, and let you switch between those load outs on the fly so you can coordinate a few power sets that work well together but swap them when other sets are more useful to the situation. 2) An interesting idea would be to use situational awareness to execute moves without specific user inputs differentiating the exact power used. For example, you could have a single boost button that uses a different element depending on if the player is on land, water, in the air or dodging (fire rocket!). And you could have a close/melee attack and ranged attack for each element that you can specify, but the exact effect/attack it creates can vary depending on the environment and enemy type of the target. Let it feel a little bit like the character is making decisions, not just you, like Batman in combat in the Arkham games. And of course, there would be a charge up to a special attack that uses the Avatar state and all 4 elements at once.