First of all, I don't want to come off as demanding. These are only suggestions to be considered.

I know abilities are split into the various elements, but perhaps there could be a whole plethora abilities to add (but you can only have 8 on your wheel at a time) with the abilities put into categories such as Offence, Defence and Misc. (healing, movement etc). The reason I suggest this is because there are so many unique abilities showcased in the series that are very distinct. I also think more items from ATLA would be cool

Item Suggestions:
Spirit Oasis Water: Although the special properties of Spirit Oasis Water was never fully explored, what we do know is that it's effective for healing, even bringing back people from the dead. (When Aang was hit by Azula, he actually DID die.)

Bison Whistle: Like wheat for cows or carrots for pigs, but it works on any peaceful mob.

Water:

Water octopus would be an amazing protection-based water bending ability. (The same ability Katara taught Aang and used in the finale of Season 2 to fight off the Dai Lee.)

Add a "wet" debuff and when a mob has it on them you can freeze them for a short time using water bending.

Pulling moisture from the air in the higher levels of abilities would also be useful. Or maybe there could be a "master" exp bar, which fills based on the experience of all of your abilities in that element and once it hits a certain point you can water bend without a water source at the expense of more Chi.

Air:

Air scooter would be an amazing transport move to add (maybe lets you move up walls, too?)

Air bubble allows you to breathe under water for longer

Earth:

Make Earth wall a little more lenient with when it lets you create a wall. Sometimes it's just not possible in the heat of battle to try and pinpoint the exact block you need to create the wall.

When Rock throw is levelled up the option to automatically pick up a nearby block could be possible.

A "vibration" sense ability that applies Glowing to mobs within a certain radius based on level.

Thanks for these good suggestions. I'll try to implement some of these abilities/items later on, as the priority right now is pushing out the a5.0 update we've been working on for many months now. But what did you mean by making Earth wall more lenient? Just making the range longer?

Thank you for the swift response!

Yeah, what I meant is that if you don't click on the top block then the earth wall won't rise up, meaning that if you're fighting on uneven terrain it can be more dangerous trying to figure out how to pull up the wall than to not use it at all.

Also, I noticed if you create another water arc while one is out it behaves really weird. And sometimes it glitches out and they start flying off automatically in a set direction or it gets stuck to you and you can't launch it. In fact, it seems quite a few abilities glitch out when trying to use multiple while another is active.

Fireball is also extremely powerful for a move that costs such little Chi. It can instantly kill a witch with a direct hit.

Just to avoid making more suggestion threads, figured I'd add my bit here
Could the pre-selection menu show the basic abilities granted by becoming a bender, and the special traits (like air's wall run and water's full moon buff, etc. etc.)
A seismic sense ability for earth that finishes off with night vision, blindness immunity, and glows mobs (upgrade: through walls)
Lava bending - wall of lava, turn stone to lava, lava to stone, lava blade, upgrade to earthbending that allows you to move obsidian
Metal Bending - item magnet, bend iron (duh), pull ingots from ore, armour type ability, cable attack (and item)
Plantbending - drain water from plants (turn area of grass to dirt and all plants to dead bush), bend leaves, vines, lilypads to make green water arc, create vine mob (rideable, you sit inside it for defense - this one's probably a long shot)
Add astral projection to airbending - pseudo spectator mode
Air scooter - functions like waterskate, upgrades to go on water and up walls

Special technique scrolls - techniques that require a special version, such as finding an airbending scroll (scooter) in order to unlcok air scooter - these scrolls also work as standard scrolls