In an interview with the creators, they call the spin-off series Legend of Korra. It will be more serious than the original series, and I'm glad. I liked the original series best when it was serious. I'm not sure I like something else about the new series, though -- It's set 70 years after the finale of the original series. All the original characters are dead. The main character is the new Avatar, a Water Tribe girl named Korra.
This means we won't be reunited with our favorite characters, but more than that, we may never learn the answers to questions left open-ended by the original series. The whereabouts of Zuko's mother, for one. The only one I can think of actually...
But we need to know that! I always expected the original series to continue in some way, if only to answer that question. The show quite purposely left us with it in mind. Was it only to frustrate us? I hope not. There's a chance the new series will answer the leftover questions. With luck...
Continuing the popular Avatar the Last Airbender series, Legend of Korra comes out in 2011, supposedly.
Here's a piece of concept art from the series.
TTFN!
