With this being a fanon set in the far past of the Avatarverse, cultures and customs will need to be explained. These won't be full run-downs, they may come at a later date on different pages.
The Earth Kingdom
The Earth Kingdom is the collective name for the twenty Earth Kingdoms which make up the land. These pieces of land can vary from being small to very large, depending on the King's influence and military power. Thus, many civil wars have been fought for money, food and power. Through all of the conflicts, there is some form of a singular governing body that helps run all of the kingdoms.
The High King: is the one with the most amount of power and land and takes his seat at the top of the Table of Stone. He holds the most sway in terms of debates and declarations of war and he is recognised as the overall 'leader' of the Earth Kingdom, though many constantly try to usurp him.
The People: The people of the Earth Kingdom are strong, stubborn and loyal to their chosen king. Since they are ruled by twenty different regimes, the quality of life differs for many; the worst areas usually being the High King's lands as they are the most densely populated. Equality is an issue during these times, but not something many pay attention to or make it a key part of their life. Peasants just want to survive, so they take any help they can get: whether it be male or female.
Disabilities are an abomination in the eyes of the earth people. They believe in a strong build and the ability for it to work for family and kingdom. Anything less and the person would be cast out, never to return again to his home.
Religion: Religion went as quickly as it came in the Earth Kingdom. Since they are a practical people who believe that the only way to solve problems are to do them yourself, they never relied on praying or deities to develop. Many do revere the badger moles who taught earthbending to the people of the lands and several memorials are scattered around where people go on pilgrimage to complete their earthbending training or to pay respect to a lost relative.
Military: The Earth Kingdom military, whilst impressive, has a significant draw-back. With each individual army swearing oaths of allegiances to their respective kings, it is hard for them to work together in a structured unit. Though a Grand Call has never been made, short-lasting truces are hard to accomplish as the armies bicker and fight within each other. The army focuses heavily on defensive positioning, utilising heavy armour which protects the user from most forms of attack during battle.
Because of this, the armies of the Earth Kingdoms are cumbersome and slow but pack a heavy hit; sometimes making the first-strike in a battle the only one before victory. Generals in the army also need to know the weaknesses of their forces and over the many years of conflict, distilled a tactical-heavy approach to every battle, with armies moving around the battlefield for days before even making the first move. This also gives the men of the army remarkable patience and endurance.
The Water Clans
The people who live at the world's South Pole have a very young form of political structuring in the form of clans. There are hundreds of clans in the tundras, but only a few carry weight and name. The most noticeable ones are Clan Wolf and Clan Bear, two heavily populated clans who have remained respectful enemies for years.
Clan Meetings: Clan meetings in the South Pole are not regular events and when they take place many become suspicious of what will happen next. This is mainly due to the hazardous conditions of traversing the land to reach another clan's village. Only two meetings have ever taken place with all of the clans present over the entire history of said clans; it is often avoided, though, due to the incredibly high death rate that happens during the processions.
The People: This society is heavily one-sided and steeped in culture with the males controlling each household unquestionably. The role of women is to heal, cook and clean whilst the men go out to hunt and, more often than not, drink. Most girls are used as bartering chips when they become of betrothal age so that a family can increase in size and strength and assure a worthy heir.
Each residence in a clan's village must pray to the animal they use as their symbol and have the head of one at the entrance of their abode. For example, members of the Wolf Clan must have the head of a wolf perched above their home or else they will be expelled to wander the deadly terrain.
Religion: This is a key part to the culture and philosophy of the clans. All people worship the sea for the food it brings them and the animal their clan is named after. Clan warriors also pray before and after battle, giving both enemy soldiers and their own a proper ceremonial burial. One of the key features of their religion is the Shaman. The shaman is always the oldest woman across all of the clans and has to abandon her name and family to move to the centre of the South Pole where people go to her for guidance. It is said the spirit of the sea communicates through her to advise its children.
Military: No formal military stands with any clan, though it is assumed that the men (sixteen-year-olds and older) of the tribe will move into action when called upon by the clan leader. Battles are often quick skirmishes that vary depending on the situations and terrain; this gives the clans very malleable combat that is always changing with no other tactics except 'charge'
The Air Kingdom
Society: The Air Kingdom is one of the most advanced in terms of a society. It has one clear leader and a council of people who's members specialise in each field of society. These range from education to military, spirituality to science. Every person in the Air Kingdom is treated equally with the Air King being the only one above everyone else. Contrary to this rule, the leaders of each sect of society are usually held above everyone else as they are relied upon to keep their respective sects in order.
The People: The Air Kingdom's citizens are very mixed in what they do and who they worship. Some may keep to themselves, such as smithies, and others may abide by the Temple Code and partake in charity work. They also pride themselves on their advancement and therefore, their intelligence. This does not go without merit, as the airbenders lead in weapon crafting and inventing, often supplying blueprints for siege machines to the Earth Kingdoms.
Religion: The main block of religious worship for the airbenders are the temples that are located on each of the Four Spires. They pay homage to the flying bison and the spirits of the wind on a regular basis; its members either shaving their hair completely (if they are male) or just their foreheads (if they are female). The High Priest Yuan holds a considerable sway in meetings with other council members, often holding the final say in some debates. This shows the power of the temples in airbender culture and on the way it influences life on the Four Spires.
Military: The Air Kingdom military is one to be revered. Though it has never really been used past holding down locations and body-guarding, it is very powerful as each member has the ability to bend. Due to this, the army has a heavy focus on attack, rarely using any form of armour or shielding for defence. Instead, they utilise airbending and evasion to get the one-up when attacked, rarely letting up until the enemy is defeated.
The leaders of the army barely have anything in experience, so they have mock battles in between the army to keep their minds sharp and their soldiers' too should the need for war ever arise.
The Fire Tribes
The Tribes: On the Fire Islands plant life is lush and plentiful, but due to the segregation of the islands it varies and so do the people who live there. There aren't so many tribes as there would be in the South Pole, but there are a few who each have different customs and regulations. Some may be a peaceful people who only eat fruit whilst others may live for battle and conquer fellow tribes in an attempt to fulfil their cultural needs. There are mid-ways in the tribes where they want neither war nor peace, so act accordingly when the need arises.
Many of the tribes settle in one place, but there is one tribe - the Sun Tribe - who are nomadic, barely staying in one place for more than a week or even a day. In the end, it all depends on what ideology the people of the tribe hold that governs their actions and lifestyles.
The People: Not much can be said about the people of the Fire Islands except they are fiercely loyal to family and their tribes, often sacrificing themselves if attacked to save others. They are a people who prefer tight-knit communities rather than large tribes, making them hard to find in the lush forests where most reside. For them, the needs of the tribe come before the needs of the individual and thus crimes are rarely committed between members of the same tribes. If one was to commit one, they are banished from the tribe and left to walk alone, brandishing a mark to show their disloyalty, assuring that no other tribe could take them in.
Religion: All tribes pay homage to the spirits in some way, blessing them for being given the gift of firebending and other things. Some people, mainly the Sun Tribe, worship the dragons and try to follow their trail to lead to a promised land where the food will never run out and peace is assured.
Military: No formal military exists for the tribes as they rarely communicate with each other. Instead each tribe has their own warriors, excluding peaceful tribes, to fight battles. They often use their home ground to get the upper hand and are adapted to fighting offensively, aiming for supplies in the first strike. Their usual strategy for dealing with a rival tribe is to make sure they cannot recover, often taking them as prisoners or stealing food and resources.