The approach to Bending that I use in my Avatar stories
Relies on two elements. There are the martial arts forms that go with tapping into each element, and there is the precise system of elemental magic that goes into the union as channeled by the forms. The very reality that different elements can use the martial arts forms of other types of Bending means that the martial arts techniques used are more specific concepts of channeling than something innately connected to those traditions.
Bending is magic, an act of conjuration, that works fundamentally differently with each of the different elements. Earth and Water rely on direct contact with their element at some point, but with each even a small amount can lead to very deadly results in the hands of a master. The most dangerous weapons in the hands of Earthbenders are not boulders but very small rocks wielded with extreme speed and bone-crunching force (essentially rock-bullets + physics). Waterbending is the most devastating type of Bending for human flesh. Not just Bloodbending, its regular manifestation with water whips can and does flay flesh to bone with a casualness that nothing else matches, while the whole 'universal solvent' factor gets turned up to 11 with magic water.
Firebending and Airbending reflect conjurations of Will and Spirit, respectively. I compare Firebenders to the Green Lantern Corps semi-derisively but given the versatility of Firebending and its connection to Will it is also a very on the nose comparison. At a societal level they also represent my usual take on deconstruction with that kind of premise, as their culture even at its best has a distinctly unhealthy and violent undertone to it. Cultivating power based on will carries a vast amount of toxic elements to it, with the Hundred Year War Fire Nation taking further what was always there, not introducing some vast aberration.
Airbending is innately spiritual and does have all the predictable drawbacks, at least with 'pure' Airbenders (which due to a variety of reasons the Avatar never qualifies as for any element due to what is involved in wielding all four of them) of purity culture. It also has that flip side in the Shunned Ones (of which canon Zaheer is a magificent example) who embrace pure evil or even pure jackassery. Purity is not good in and of itself and when it goes wrong it goes very, very badly wrong.
And like Firebending it does not need actual air around it to sustain itself, as both are rooted in the Soul (which is in its own sense exactly why the Fire Nation targeted the Air Nomads first. They knew that the other element that didn't have the drawbacks of the other two could indeed validate Aang's view that awareness of the genocide would have prevented it from being fully carried out and with the Air Nation involved collectively from the start it would have been a very different and much shorter war).
This is one of the big reasons why the various Nations ultimately find means to impose hegemonic magical traditions (but there are also always exceptions, as the Sun Warriors relative to the Fire Nation, the Foggy Swamp Tribe, and the Shunned Ones all show). It's because Bending at its core grants the individual Bender power at the highest levels to fit in good company in settings like DC and Marvel and 40K which are pretty damned powerful all by themselves and the traditions exist to ensure this power is wielded responsibly and in a controlled pattern.
