Fire. Air. Water. Earth.
With the 20th anniversary of Nickelodeon's Avatar: The Last Airbender upon us, creators Michael DiMartino and Bryan Konietzko announced the a brand new Avatar series!
Avatar: Seven Havens: A 26-episode series split into two 13-episode seasons following a young female Earthbender who is the next Avatar following Avatar Korra.
Cannot say I like the devastating cataclysm part, especially if it makes the Avatar a symbol of destruction, but a new Avatar series is cause for CELEBRATION! I, at least, was and remain a huge fan of Legend of Korra and can only hope that Korra's legacy and memory is a strong force for hope in the world. Although, given the post-apocalyptic circumstances, I have a very bad feeling that that is not the case and that the Spirit Portals are to blame. Hence my chief hope is that the new Avatar has the same, or similar, relationship with Korra that Aang had with Roku.
As to the question of why Michael DiMartino and Bryan Konietzko decided to level the Four Nations, I can only speculate that - from a storyteller's point of view - they needed to make Seven Havens significantly different enough to stand on its own two feet so as not to be viewed as an inferior copy of the previous series. Which is exactly what they did with Legend of Korra and why, I think, the series is so good. I know some have problems with Legend of Korra because it was so different but to me its differences are what made it amazing. It was its own show, a sequel of The Last Airbender without being clone, taking the world and story forward in ways that made sense (people who complain about all the modern technology clearly forgot how advanced the Fire Nation was during the Hundred Years War and the submarines Sokka built).