Most details are in the article ^
Reddit source of images: https://www.reddit.com/r/TheLastAirbender/comments/1hi2tte/more_confirmation_on_the_leaks_this_was_using_the/
I find this interesting! Post apocalyptic is a good way to “reset” the world, and the idea of twin Avatars has been batted around the fandom for some time.
That sounds amazing and perfectly in line with the canon!