A long time ago, Aang became enlightened about a certain divine truth.
There is no boundary in bending.
This incredible, powerful fact was not known by any Avatar previous to Aang; he knew this because he took the time that he had to get to know the five hundred and sixty-two Avatars that came before him.
Aang doubted that even Pathik knew the sheer depth of the illusion of separation that plagued the Chakra of Light. Nor did he think that Pathik knew just how appropriate the name of that Chakra was.
He came to understand the truth in this way:
Firebending… In truth, the bending discipline of energy and light. Fire, lightning, light, chi, even magnetism would bend to the will of a firebender. It is the most fundamental of the four disciplines… it touches the purest expression of existence in the universe. All things are made of energy (the energy present in an object is equal to the mass of the object multiplied by the square of the speed of light…), therefore, if a firebender has significant control and imagination…
Earthbending… In truth, the bending discipline of matter. What makes up the Earth? He pursued that question once with a scientific curiosity, and was subsequently astounded by the variety of the substances which made up the earth. If stone can be bent, so can crystals; if crystals can be bent, so can minerals; if minerals can be bent, so can metal and ice; if ice can be bent, so can water. And so on and so forth, ad infinitum…
Waterbending… In truth, the bending discipline of motion and dynamic flow. Push and pull, yin and yang. The true essence of the discipline is that for every action, there is an equal and opposite reaction; it is not about the water, but the tides. A good waterbender that can grasp this would know then that any fluid can be bent... even air. Heat is merely an extremely small version of kinetic motion; tiny, unbreakable building blocks of matter striking and bouncing back. Freeze, unfreeze… heat up… hotter…
Airbending… In truth, the bending discipline of transience. Here today, gone tomorrow. Things do not work the same way when they are extremely small… things can exist in more than one place, things can move almost freely across time… they can even appear, or disappear. These effects become more infrequent as the scope of observation grows larger… but what is that, to an airbender, who can tell the wind to blow against its source?
And, eventually, finally, ultimately… Aang came to the conclusion that any bender of sufficient skill should be able to bend anything.
Knowing these secrets wouldn't be enough, though. It would require a deep familiarity with the way the universe works… So deep, in fact, that it would take several lifetimes to cultivate it; then a few more lifetimes of practicing the bending.
And Aang smirked as he stood upon the pillar of stone, staring across the chasm way at the man who had the audacity to call himself ruler of the world.
How can you rule something that you don't understand in the slightest?
Ozai seemed considerably more worried about Aang's presence than he was in his first life; he wasn't smiling like before, for starters, "You have been a thorn in my side, undoing the work of generations in the span of months! Finally, the universe delivers you to me as an act of providence!"
The Avatar's grin merely widened, "You know what? I'm so glad that you exist. Do you have any idea how cathartic this is for me?"