I have decided to devise a long list containing a bunch of fancy theoretical bending techniques. This list is unique and made from ones I thought of on my own. Please note that a lot of these are probably too extravagant to be possible in any way, but I've tried my best to explain them thoroughly. If you wish, you can say if any stand out in the comments section. I do think some are cooler or more plausible that others. Also, feel free to give me advice for the descriptions or give me new ideas for even more techniques, which I may add as time moves on. So..... yeah, here it is:
Soundbending - Air: An airbender's manipulation of sound waves passing through the air. Since sound waves are created by the movement of air molecules causing a collision with other particles, airbenders can theoretically change the patterns of these sound waves by controlling when and where the air molecules collide. This could become very useful as a battle strategy. Airbenders can change enemy conversations at will by altering sound waves reaching their ears, confusing them or leading them into a trap. They can also make enemies go deaf by focusing amplified sound waves in their ears, giving them a major battle disadvantage. Only the extremely skillful can perform this high-level technique.
Metallic Bloodbending - Earth: A subset of metalbending, metallic bloodbending refers to an advanced earthbender's control over metal found in the human body, primarily iron (which is common in blood). Although there are only small traces of metal in humans, this technique could possibly be combined with bonebending, the sum of which may be enough to make humans a bendable substance. This would look similar to bloodbending. Only master earthbenders can perform this high-level technique.
Brainwave Bending - Fire: The human brain consists almost entirely of billions amongst billions of neurons, all of which are connected to other neurons. These neurons transmit processed data in the brain to other neurons via electrical impulses and signals. Theoretically, firebenders that can redirect lightning should be able to control these electrical charges. This dark technique could be used for a wide variety of purposes, ranging from hypnosis to a complete shutdown of the brain entirely, killing the victim. Firebenders can change or erase the information that is passed from neuron to neuron, thus, in a way, allowing them to control the victim as if he or she was just another part of the performer's body. This technique can be used on human nerves as well, as they are also dependent on electrical charges. Controlling nerves would look like bloodbending, but would also cause intense pain and nerve damage to the victim, which may become useful in torture. Brainwave bending requires the performer to be very close to the victim's body, or else it will not work.
Chibending - Fire: The act of removing chi from a bender and giving it to another person (doesn't need to be a non-bender) through a three-way energy transfer. Therefore, the person who gets the chi receives the victim's bending abilities, possibly being able to bend two elements now. This idea was inspired by lightning redirection, which is the transfer of electrical energy. If it is possible to transfer electrical energy, then it should be possible to transfer any other type of energy, including chi. This technique would be performed in the same fashion that lightning redirection is done, as they are both energy transfers. When the chi is passing through the air to reach it's destination, it will look much like lightning too. The victim will be removed of all powers when this happens.
Soulbending - Energy: The removal of one's soul from their body via energybending. There are almost no canon clues to suggest how this would be possible, but I nonetheless decided to put this up here because I found the idea interesting. The technique is nearly identical to energybending, but goes past merely the removal of chi. This form of energybending strips a body of it's soul. Although powerful, it is extremely difficult to perform and easy to die while doing this, as the performer's soul might accidentally be removed as well. Soulbending can be used to bring the dead back to life by placing a soul, or it can be used to turn inanimate objects into live creatures.
Vaporbending - Water: Waterbenders bend the vapor in air. Please do not confuse this with Hama's form of vaporbending; the two are not the same. Hama turned the vapor into liquid, while this form requires the vapor to remain gaseous. It looks much like airbending, but only with one gas in the air.
Glassbending - Earth: Earthbending the impurities within glass much in the same way metalbending is performed. Glassbending is possible only because glass is actually refined sand, which is easy to bend.
Sunbending - Air: Since the Sun almost entirely consists of bendable gases like hydrogen, helium, and oxygen, airbenders can change it's shape or annihilate it altogether. This cannot be done from Earth, as the Sun is much too far away. It would require a very large number of professional airbenders to go up into space via spacecraft and perform it together.
Cometbending - Earth: Comets are, in essentiality, just large clumps of space rock that burn as they pass through the Earth's atmosphere. Therefore, earthbenders that can come close enough to them as they pass by the Earth can theoretically alter their orbits or destroy them. Looking back at The Last Airbender series finale, had Azula not fired her Dai Li agents, she could have ordered them to get on an airship, fly to Sozin's Comet, and change it's orbit so that it went around the Earth for eternity. giving firebenders unlimited power forever.
Earthbending Quake - Earth: All earth is connected, so earthbenders have total and complete access to all of it at any time they are on the ground. By this logic, earthbenders could theoretically trigger an earthquake anywhere on Earth, no matter how far away they are from that place. Benders like Toph would be especially good at doing this because they feel around the Earth with their feet. They can channel all their bending energy to any one place and create a massive earthquake there, obliterating everything.
Woodbending - submitted by The Cabbage Man: A Hashirama-like technique, Woodbending is a very unique form of bending that stands separate from the four Classical Elements in the Avatarverse. It can allow benders to make trees, plants, and entire forests appear out of thin air. Woodbenders can also generate nutrients in the soil, which can allow trees to eventually grow. Waterbenders can use water found in trees for their own purposes. However, any trees that waterbending is used on will die.
Vacuum Bending - submitted by an anonymous contributer: An airbender could bend all the air out of a certain point, making a space that contains no air at all as long as the airbender is holding the air out. It could be used to "subdue" people by suffocation, with the added bonus that it's completely undetectable.
Woodbending II - submitted by A True Fire Ferret: Woodbending could also technically work under a sub-division of Water. In "The Swamp" we see people bending vines. Later, in "The Puppetmaster", when Katara explains plant bending to Hama, she is replied with "You can take it even further"; then Hama pulls the water out of flowers. Later, Hama is seen bending water out of a tree. Well if removing the water from something is "Taking it further", then wouldn't you be able to control a tree? And wood is made from trees, so wouldn't you be able to therefore bend wood?
Suffocation - submitted by an anonymous contributer: A very dark form of airbending, Suffocation involves the manipulation of air entering and leaving the lungs, which can be used to choke a person by making sure they cannot breathe. It can also be used to cause the collapse of lungs and immediatly kill the victim.
Light Bending - submitted by Brozoco: When a person can bend the light on Earth, giving them the ability to change the color of something or turning something invisble by taking the light away. They can also create light to see in the dark or to blind enemies.
Darkness Bending - submitted by Brozoco: The ability to bend the absence of light (darkness or blackness). By extension you can copy shadows of items and living beings to create shadow copies of something. Darkness Benders can also take away light to make things become completely covered in darkness.
Gravitybending - submitted by Flameguider: Gravitybenders can manipulate attractive and repulsive forces, letting them attract and repel anything and manipulate gravity. If a water, earth, or firebender attacked a gravitybender, they could just manipulate the gravity around them, easily repelling the attack. Like how airbenders, waterbenders, firebenders, and earthbenders can go airbourne, so can a gravitybender. By lowering the gravity level around them, a gravitybender can nearly jump in the air and walk in the air freely. Gravitybenders also have a time limit they have to wait before doing another move, which can range from 2 seconds to a year (using gravitybending to move the Earth can cause a gravitybender's inability to gravitybend for a year because of the stress on their body).
Dust Puppetry - submitted by RandoMaster07: Earthbenders have the ability to create dust clouds and manipulate them. Using their ability to manipulate the clouds of dust they could coat their opponents and allow them to control their movements. Similar to bloodbending, if the opponent is a strong enough earthbender they would be able to break free of the user's hold.
Plasmabending - submitted by AvatarCorin34: A modern technique (in the future generations after Avatar Korra) in which a fire bender can manipulate and create plasma (when it hits something, it creates an explosion). The bending style is waterbending inspired with it's fluid motions.