In a previous post, I explained my complete theory on what would occur in 14 and 15. A lot of the stuff was sketchy,but I had an idea that the "new villain" would be the the Warden of the Boiling Rock. As I said in that other topic, I've based this on the fact that 16 is called "The Southern Raiders."
Bio - The Warden could be a former Fire Nation general, who was responsible for conquering the Southern Water Tribe. (As we saw in episode in "The Puppetmaster", the Southern tribe could've been a formidable place.) Due to his success, Ozai made him the leader of the Boiling Rock.
"Disturbing Connection to Katara and Sokka" - It is posssible, that in one of his final invasions, The Warden murdered Katara's mom. From that point on, Katara and Sokka acould never forget him.
Part In Story - In 14 or 15, Azula, Mai, and Ty Lee could be there to see the Warden. Due to his success with the Southern tribe, Ozai wnats him to assist in the destruction of the North. (On the day of the Comet) After the Gang battles Ozai's Angels and gets away, the Warden could be intrigued by how crafty the Avatar (and his firends) are. (At some point in this episode, Katara could come in contact with him, setting up a flashback for the next episode.) The Warden, with a new motivation for destruction, could then leave the prison to regroup his former Invasion team. The infamous Southern Raiders.
Reasons I like This:
I'm REALLY REALLY tired of nothing but Uber-Azula fight scenes in these episodes. (She's good, we get that) I want a new villain become a recurring character.
His group of Southern Raiders, could be team of really cool, individuals villains. (Like they could all have different weapons and traits) They'd keep things exciting for the final.