Thanks to we now have the description for The Rift Part 3.

The Rift Part 3 Cover

Team Avatar is in peril as a dangerously powerful ancient spirit awakens with vengeance and destruction on its mind. Bonds will be tested and new truths revealed as Aang must answer an important question: Can humans and spirits co-exist?!
Interesting that the book will deal with the idea of if humans and spirits and co-exist when Korra Book 3 will be covering the exact same thing. Other than that it is fairly general and non specific as expected for a final part. Either way it seems like The Rift will have an exciting end. Bonds tested and new truths, I wonder what these will be. Thoughts!