You never know how things are going to work out. This answer is subject to change.

Aang

• As the Avatar, Aang worked with both Fire Lord Zuko and Earth King Kuei to restore Earth Kingdom sovereignty over its territory. Both parties eventually developed the Harmony Restoration Movement, which Aang ensured went according to plan.
  • Along with Katara, Aang helped settle an impasse between Fire Lord Zuko and Earth King Kuei over the Fire Nation colonies.
    • Driven by anger upon seeing the members of The Official Avatar Aang Fan Club tattoo themselves with the traditional and sacred Air Nomad arrows, Aang became blinded about the fact that the nations must be remain separated to maintain balance.
    • After listening to Katara's confession about the Movement and her advice, Aang set off to confide with Roku about what he should do. Roku confessed his relationship to Zuko and insisted that he fulfill his promise.
  • Aang appeared in the middle of the battle and showed Earth King Kuei that he was not only fighting the Fire Nation, but the other nations, including his, as well. This ended the conflict between the Fire Nation and the Earth Kingdom forces as both the Earth King and Zuko agreed that both the nations could coexist peacefully.
• Aang formed the Air Acolytes from the members of The Official Avatar Aang Fan Club; they eventually played a role in Avatar Korra's life.
• Sometime after the war's end, Katara and Aang married. Eventually, they parented three children: Kya, Bumi and Tenzin.
• Aang went off to search for any remaining traces of the sky bison and winged lemurs, and eventually found surviving breeds. Such a sky bison is Tenzin's own Oogi.
• With Fire Lord Zuko, Aang transformed the Fire Nation colonies into the United Republic of Nations, the capital being Republic City.
  • In Republic City, Aang established Air Temple Island, which became the center of Air Nomad culture in the Avatar World.
• Aang was involved in the trial of criminal malefactor Yakone. Aang used energybending to remove Yakone's bending abilities as his renegade bloodbending was a threat to the safety of Republic City.
• Prior to his death, Aang appointed the Order of the White Lotus to protect his reincarnation.
• Aang died at the biological age of 66. Avatar Korra was his successor.

Katara

• Katara assisted in carrying out the Harmony Restoration Movement, by helping transport Fire Nation colonials back to the Fire Nation.
  • Along with Aang, Katara helped settle an impasse between Fire Lord Zuko and Earth King Kuei over the Fire Nation colonies.
  • Katara explained to Aang that she changed her mind about the Harmony Restoration Movement because when she saw Kori's family, she also saw her future with Aang.
• Sometime after the war's end, Katara and Aang married. Eventually, they parented three children: Kya, Bumi and Tenzin.
• After her husband's death, Katara became the waterbending master of Avatar Korra, Aang's reincarnation.
• An 85-year-old Katara was last seen in the Southern Water Tribe.

Zuko

• As the Fire Lord, Zuko worked with both Earth King Kuei and Avatar Aang to restore Earth Kingdom sovereignty over its territory. Both parties eventually developed the Harmony Restoration Movement.
  • A failed assassination attempt on Zuko was conducted by Kori Morishita a year after the movement's proposal.
  • Zuko visited the town of Yu Dao, the oldest Fire Nation colony in the Earth Kingdom. After witnessing the cultural immersion between both nations, Zuko removed his support from the Harmony Restoration Movement, leading to a conflict between him and the Earth King.
    • Zuko led his army to Yu Dao in order to protect his nation's citizens from being removed from the colony because of the Harmony Restoration Movement, and was met with opposition from the Earth King's army. However, due to Avatar Aang's intervention, both he and the Earth King agreed that both the Earth Kingdom and Fire Nation citizens could coexist in the colony.
• Believing that it would help him connect to a part of his heritage that wasn't so confusing, Zuko turned to Azula to help him find his mother, Ursa.
• With Avatar Aang, Zuko transformed the Fire Nation colonies into the United Republic of Nations, the capital being Republic City.
  • Zuko commissioned for a statue of Aang to be built in Republic City.
  • A statue in Zuko's honor was built at the Central City Station in Republic City.
• Eventually, Zuko had a daughter, who acquired the Fire Nation throne in 167 ASC after Zuko stepped down as Fire Lord.
• Zuko had a grandson named Iroh, who later became a general of the United Forces.
• Zuko continued to travel the Avatar World as an unofficial ambassador for balance.
• Zuko is currently 87 years old.

Toph

• Sometime after the war, Toph founded the Beifong Metalbending Academy to teach the specialized art of metalbending.
• On the day of the Battle for Yu Dao, Toph, under Sokka's command, disabled all of the Fire Nation's invading tundra tanks by metalbending the screws in the tanks.
• Toph served as Chief of Police for Republic City's Metalbending Police Force.
• Toph was involved in the trial of criminal malefactor Yakone. She and her police force were present at Yakone's arrest and subsequent trial.
• Eventually, Toph gave birth to a daughter, Lin.
• Toph died at an unknown age. A statue in her honor was erected at the police headquarters in Republic City, where her daughter became chief.
• She died sometime prior to the events of Avatar Korra's lifetime.

Sokka

• Sokka assisted in carrying out the Harmony Restoration Movement, and was the one who came up with the operation's name.
• Sometime after, Sokka joined Toph at her metalbending academy and helped prepare the class against a rival school.
• During the Battle for Yu Dao, Sokka was the mastermind of the plan to disable the Fire Nation tundra tanks with Toph's metalbending.
• Sokka served as Chairman and Southern Water Tribe representative for the United Republic Council.
• Sokka was involved in the trial of criminal malefactor Yakone. He was Chairman of the United Republic Council at the time, and delivered Yakone's guilty verdict.
• He died sometime prior to the events of Avatar Korra's lifetime.

Suki

• Suki and the other Kyoshi Warriors were appointed by Mai to serve as Zuko's guards following the failed assassination attempt.
• Her revelation to Mai that Zuko snuck out at night to visit his father in jail led to Mai breaking up with him.
• Suki asked Toph and Sokka to help her stop Fire Lord Zuko's army on the day of the Battle for Yu Dao. Toph and Sokka assisted Suki in stopping the Fire Nation's tundra tanks.