The bending arts in Avatar: The Last Airbender are rather rigidly defined — a few people can manipulate Earth, Air, Fire, and Water, and fewer are able to wield exotic sub-species of these disciplines, such as Lavabending, Lightningbending, and Bloodbending. In fact, Metalbending only comes into existence long after the other versions, thanks to the incandescent brilliance of Toph Beifong.
That being said, what could be the other forms of bending that would make reasonable sense in the Avatar universe? Should these new variants be related to the original four, or can they be entirely independent? Is it possible for a bending style to be powerful enough to affect even the Avatar?
10 Lightbending: Controlling Electromagnetic Waves
Lightbending could theoretically be a sub-type of Firebending, but it also works as a totally independent style. Controlling light would mean controlling electromagnetic waves in general, from X-Rays all the way to Radio Waves, and the stronger the bender the more range they'd possess.
Light is meant to travel in a straight line, according to the rules of physics, so manipulating it is of immense advantage. For instance, one could increase or decrease visibility according to the situation, such as creating an aura of darkness for stealth purposes. Also, there is a small chance of surpassing the rigidly defined speed of light.
9 Gravitybending: Forcing Targets Down To Earth
Gravitybending goes hand in hand with Earthbending, but is likely a considerably stronger technique than its parent. Imagine forcing an opponent's knees to buckle in the middle of a fight, as if their body is glued to the ground, preventing movement, similar to Bloodbending.
On the extreme side, Gravitybending could be employed to pull airborne objects towards the earth, such as airships or flying Airbenders, and the most talented practitioners might even be capable of generating a micro black hole.
8 Plasticbending: Controlling Everday Objects Made Of Plastic
With all the advancement in technology that has happened between the show and its sequel, the invention of plastics is either on the horizon or has happened by now. Being a completely new material, Plasticbending would likewise be in its infancy at first.
However, it shouldn't be long before enough people are born with the ability to form a society around it, as the four main bending disciplines have. A Plasticbender can probably exploit the innumerable instances of plastic used in daily life, including bags, automobiles, electrical devices, and so on. On a different note, they could stretch their own bodies as if made of elastic.
7 Woodbending: For Building Structures & More
It's surprising that Woodbending isn't already a thing in the narrative, given that wood is part of the Wuxing, or the five elements commonly referred to in Chinese philosophy (which has had a massive influence on the worldbuilding process.)
Besides, this substance is commonplace, be it in living form or structural frameworks: the fact is that most houses are built from wood, not to mention all the paper obtained from its pulp. In addition to accessibility, this version of bending can be modified in quite a few ways, making it versatile.
6 Soundbending: For Eavesdropping & Throwing Voices
As a subsidiary of Airbending, because it's all about air vibrations and their movements, Soundbending might not be very powerful, but there is no doubt that it has its uses.
For example, if one wanted to eavesdrop on a conversation taking place a great distance away, they would merely have to redirect the sound waves from the specified location to their ears. Similarly, Soundbenders could throw their voices, like ventriloquists, but with a lot more precision and elegance, so as to avoid detection or confuse the target.
5 Radiationbending: Power Generation With Precaution
The concept of nuclear physics is presently undiscovered, but it's only a matter of time before an Einstein or an Oppenheimer emerges to introduce society to the wonders (and destruction) of radioactivity. In terms of practicality, Radiationbending is applicable in the fields of power generation, as in the real world, because of the relative safety of its use and its superior efficiency.
The problem is that radioactivity is unnecessarily harmful to living beings, even if Radiationbenders are immune to its negative impact, so a lot of precautions would need to be taken. Not to mention the potential dangers of the inevitable atomic bomb.
4 Spacebending: Transporting To A Hidden Zone
Spawning a vacuum sounds cool, but only after the safety protocols for Spacebending are firmly established and ingrained in the minds of future practitioners. Sucking an enemy combatant into a hidden zone known only to Spacebenders would be an incredible way to win the fight, especially without hurting or killing anyone (assuming that they have the basic necessities to survive in there.)
In other scenarios, refugees and others escaping from dangerous situations could obtain sanctuary in these locations, although multiple Spacebenders might be required for this.
3 Plasmabending: The Most Violent Form Of Firebending
The most violent extent of Firebending is Plasmabending: the creation of a "flame" so hot that it would be able to melt or incinerate nearly all materials in existence. For context, the average temperature range of fire is 1000-3000 °F, while plasma hovers between 10,000 and 15,000 °F.
One can argue that lightning, being approximately 50,000 °F, is a preferable technique but Lightningbending is highly risky and its effects don't last very long. Plasmabenders are liable to be highly sought after for the sheer force of their skill, much like Combustionbenders are.
2 Mindbending: Manipulation Of An Individual's Mind
The manipulation of another's thoughts is most definitely one of the greatest forms of bending that can be added to the Avatar canon, for obvious reasons. Like the Imperius curse in Harry Potter, Mindbending can be utilized for actions as mild as gently prodding an individual into taking a particular decision to literally converting them into the user's puppets for all intents and purposes.
Therefore, Mindbenders are expected to be held accountable for their practices, although autocratic organizations may unfairly suppress them as a preventive measure (which would not end well, for sure).
1 Timebending: Timetravel & Altering The Flow Of Time
Leaving aside the probability of paradoxes (the standard outcome of messing with the sequences of events), Timebending is a magnificent tool that has limitless potential. Traveling too far back into the past or ahead into the future is certain to cause unwanted ripples in the timeline, but taking a hop into yesterday or tomorrow should be feasible.
This way, a Timebender won't have to rely on the irreversibility of temporal flow and can make their choices accordingly. There is much that can go wrong with this discipline, though.