i can see a sequal animated movie coming out similar to say full metal alchemist, because there are some weird cliff hangers in the ending episode that would have been rapped up if there wasnt something like that in mind. For instance zukos mother could have easily been confirmed as dead but they left it with us not knowing what happened with a very cliffhangery moment. Plus we do not know what happened to Azula. There is also the whole question of will aang rebuild the air benders. Lastly in a show about uniting the four elements, with seasons for Water, Earth, and Fire, but no Air. It seems that they are lacking something. So it would make sense to have a last Avatar:The Last Airbender straight to television animated movie that would take the place of Air and rap up the last few strings.
I am not saying they are doing this. But it makes allot of sense.