Avatar Xiwang
500 years after avatar Aangs death, an avatar from the fire nation was killed in the avatar state, everyone assumed that this meant the end of the avatar. About 50 years has passed since then, and the world has broken out in war, since the invention of guns, there are a people who call themselves the 机械人 (PoM) meaning "People of machines," they broke out in war against the four nations. All hope was thought to be lost since there was no more avatar to bring balance to the world.
In the Southern Air Temple, the nomads had repopulated enough so there could be once again nomadic people practicing the art of airbending. Everyone in the temples was a bender, except one boy, named Xiwang. Xiwang had been the only person born in an Air Temple to not be a bender. He just for some reason could not bend. But then one day when a small group of PoM members decided to scare a few of the air nomads. They fired warning shots at the temple, then came up to the gates. Xiwang was standing there and they saw him, they told Monk Youmou, the airbending instructor, to bring him forth. They pointed a gun right at his head, and just then, Xiwangs eyes began to glow, he made a dome out of earth that separated the PoM, and the temple, and he then shot fire at them, forcing them to flee. When his eyes stopped glowing, he fell with a thud. The monks helped him up and didn't know what to think, the only person who could have done that was an avatar, and the last one died just over 50 years ago. They had a talk with Xiwang, they asked him what happened to him when he was in that state. He replied by saying that a man who wore a robe of fire took him somewhere, and told him that he was the avatar, but he was an earth bender. He explained at how when he, the past avatar, was killed in the avatar state, it didn't end the cycle, it mixed it up. He told him that he must master the 4 elements starting with earth, and then take down the PoM.
After the monks heard this they called the Earth King, they said that the avatar is back, and he's an earth bender, but was born an Air Nomad. The Earth King agreed that he shall start training immediately, one month later he said his goodbyes, and was off to the earth kingdom.