War. War Never Changes.
The fall of the world's Nations went beyond all expectations. In a few hours, the Fire of a thousand nuclear warheads scorched the world of its greatest cities, burning away tens of millions of lives. The Air carried the radiation across the world, even to the smallest villages far removed from any ambitious kings or generals. The Water fell from the clouds, each drop of rain bringing enough poison to last generations. And the Earth, long trusted as a source of sustenance, held the radioactive wastes for generations, refusing any attempt of cleansing.
Only one figure could have protected the world in this time of need. A great bringer of Peace, a medium between Peoples and Nations, and Avatar of Harmony. Either for the violent world before the War, or for the desperate one that came after. But when the world needed their paragon the most, they vanished. And soon, we began to doubt they ever existed.
We couldn't say the same for other seemingly supernatural beings. The Spirits that once lived alongside us, part of the natural order of our World, sought revenge for the Order's collapse. Every spirit lost something back then, and even those who don't conspire against us will look the other way when their brethren seek to destroy us. And so, even as we sought peace between survivors, there could be no peace between worlds.
A hundred years have passed. My brother and I have rediscovered the lost hope for humanity, an Airbender named Aang. His Airbending is Incredible, and Desire for Peace Infectious, but he is still young. Still a naive child.
Perhaps that's why I believe he can save our fallen world.
I'm sorry. I tried making something sillier, preferably with Kirby, but this popped into my head and wouldn't leave. I'm thinking you take the more advanced tech in Korra to its logical conclusion (Nukes), and make a Fallout-like setting with the fantasy elements of the Avatar universe. The result would be a sort of synthesis between the themes of the two series. Fallout games are about hukmanity's constant enduring nature while Avatar is about restoring Peace to a damaged world.
