Avatar: Legend of Korra
under that working title, it's been confirmed that since the original Avatar series was such a huge success, theyve decided to continue the story
it's supposed to begin in 2011 and the original co-creators are involved
from the Avatar Wikia:
Avatar: Legend of Korra takes place 70 years after Avatar: The Last Airbender. Korra is the next Avatar after Aang, a rebellious girl who is hotheaded, independent and "ready to take on the world". Korra comes from the Southern Water Tribe, and Aang and Katara's son Tenzin is her Airbending teacher.One of the story's settings is Republic City, a "metropolis powered by steam-type technology", "inhabited by people from all nations". Here Korra must deal with rampant crime and an anti-bender revolt. Mike and Bryan have said that the world of Avatar has "definitely changed, and evolved, and advanced, but we're very conscious of keeping the same feeling. Not totally different, but it's definitely generations later."
for more info, check out the wikia: http://avatar.wikia.com/wiki/Avatar:_Legend_of_Korra