Not sure if all these are true or rumors, but from what I know this is what might happen in Book 3.
First of all Book 3 will be titled Change which makes sense since opening spirit portals is a huge change for the world either positively or negatively (not known yet, but I'm leaning towards negative side).
It is also known that more about Earthbending. What we will learn I am not sure of.
We will also be getting more of Lin Being Fong as she was not seen too much in Book 2.
Book 3 is nearly complete but the release date isn't confirmed quite yet. I hope that Book 3 will be much more interesting than Book 2 which I didn't enjoy all that much.
That is all, please comment on what you want for Book 3 and your opinions.