More information was leaked, alongside a lovely piece of concept art :]
http://www.toonzone.net/news/articles/34425/new-series-avatar-the-legend-of-korra-premieres-in-2011
If you don't want to read the whole article, here's the most important paragraph :]
The Legend of Korra takes place 70 years after the events of Avatar: The Last Airbender and follows the adventures of the Avatar after Aang – a passionate, rebellious, and fearless teenaged girl from the Southern Water Tribe named Korra. With three of the four elements under her belt (Earth, Water, and Fire), Korra seeks to master the final element, Air. Her quest leads her to the ep