Setting: The Four Nations. Need a map? Look one up on Google.
Disclaimer: I do not own Avatar: The Last Airbender (sadly). I simply made up this plot while bored.
Forty years had passed. Forty long, hard years. Avatar Aang had married Katara, had three children. Sokka and Suki were happily living together on a rebuilt Kiyoshi Island with the rest of the Kiyoshi Warriors. Toph became a hermit, only known as the Mystic One to those who sought her. And Prince Zuko was now Fire Lord Zuko, a grandfather with many strong children. Everyone was happy. Or so it seemed.
Thirty years into Zuko's rule, word of a rebellion came to him. He immediately sent word to Aang, asking him for his assistance in yet another war. Help was given, and the rebellion put down before it could begin. Zuko began to relax. He wasn't exactly young anymore. Ten years passed peacefully, with minimal conflict between countries. Zuko struggled to keep the peace, and it seemed to be working.
In the shadows, the group that Zuko had put down began to reform. They called themselves The Night Watchers, and their mission was to free the world from the reign of who they thought to be the tyrannous Fire Lord. They spread their influence throughout the Four Nations through a series of tunnels, and none of the governments know about them.
The Night Watchers have located four very special children amount the Nations. They have abilities not known to man before, and are soon scouted. Unfortunately, not everything goes as planned, and three of the now-teenagers escape. The fourth is kept within their ranks, trained as one of them. Five years after their escape, they finally informed Fire Lord Zuko of what happened, and have encouraged him to take action.