I reckon they ended it a bit too perfectly and there is not much scope for a new season. However, they could have "mini seasons" of between 2-5 episodes or Tv movies focusing on each character.
The most obvious one would be Zuko looking for his mother and how he copes with being the new Fire Lord.
Aang on helping rebuild the world, etc.
I cannot see how they can make a new season following from the previous ones and the above scenario is the only way they can continue without taking the show downhill, as there is minimal risk attached. For example, if Azula and her father escape, gahter followers, etc - on paper may sound great, but unless executed perfectly, may fall flat on its face and ruin the whole Avatar series.
Another way is if they create a spin off show - like a story about an Avatar from the past or future with completely new characters, but the mini series idea is better in my opinion as it focuses on the characters we have grown to like during the show!