Chi

Chi is the pool from which benders draw to use their bending, similar to a sorcerer in regular 5e with a mana pool. Certain moves require an amount of chi to be used, whilst others, at a sufficiently high level, require no chi.

Chi pool
A character's chi pool is determined by multiplying the sum of their intelligence and wisdom by three, hence the maximum naturally achievable chi pool is 120.

"Exceptionally powerful characters such as the Avatar in the Avatar State, or Firelord during Sozin's Comet may have a larger chi pool, reaching up to a maximum of 180."

Chi Regeneration
Chi regenerates at 1d6 per turn of combat (i.e. per six seconds of combat), under normal conditions. If a character focusses on regenerating their chi, their chi regenerates at 1d20 per turn, under normal conditions.

Bending Levels
"To make bending fit more easily into D&D, it seemed apparent to give benders a chi pool to draw from. Using the traditional spell system didn't seem to match thematically."

Benders can be in one of four levels: novice, adept, master, or grandmaster. These can also be considered to be the pre-nominal title of a bender, denoting prowess (e.g. grandmaster firebender; adept waterbender) Each of these levels corresponds to a quartet of levels, that is:

Each level corresponds to a different chi cost for moves. For example, the firebending movement move, 'jet propulsion'. To best explain this, I will use an example situation.

"Suppose four heroes, all firebenders, have just learnt the ability, and are going to travel a distance, using all of their chi. The difference between them is that each one is a different level: novice, adept, master and grandmaster respectively. How far should each be able to fly? My proposed solution is as follows:" Currently, these are the numbers but they are placeholder, to a degree. The numbers will be refined, but this is roughly what the numbers will look like for 'jet propulsion'. I feel this makes sense as it prevents novices from being too strong for their level (considering this is a master level skill anyways), and allows the best grandmasters to fly quite some distance.