3. More Airbending
This might seem like an odd one, but even after four books of the show, it feels like we still don't know all that much about Airbending. We've seen some pretty impressive examples of what's possible with the other three elements, but that's because there are so many earth/water/firebenders. In Avatar: The Last Airbender, there was only Aang, and he relied on the other elements as well. Tenzin has shown off some of his skills in The Legend of Korra, but thats about it. It would be a lot of fun to see some Airbender sparring, especially if we get to see the Airbender kids grow up.