After the war the Avatars World had many changes. Depending on its four Nations.
The two water tribes have had a few changes after the fire nation fell.
Southern Water Tribe
The southern water tribe was rebuilt with a population of waterbenders. The villages had adopted a grid pattern in their villages while the USA was rebuilding the world after the new USA-Fire Nation War.
Northern Water Tribe
The USA helped the Northern Water tribe rebuild and the tribes leader allowed Women and Men to learn waterbending togeather.
After the war, the Earth Kingdom was divided into sixteen separate city-states and the inner walls of Ba Sing Se, which help "Maintain order" were town down and Ba Sing Se became one large city. The Dai Li also became policemen and the city established its own bending academy.
The Fire Nation became a democracy after the war. It first president (Zuko) ordered that all fire nation history books to be confiscated and burned and all teachers arrested. The history books were rewritten anf new teachers were placed in all Fire Nation schools.
Because of the United Stateses cloning technology, the entire population of Airbenders, Flying Bison and lemurs were restored.