I've renamed the thread to a more applicable title. But, as Omni said, in future, please try and choose an appropriate name to begin with. This will help other users when they are looking through the list of threads to know what it is about, and whether they may wish to contribute.
eh...balance is kinda the deal with this book. and i think each title has to be a physical thing, not a concept. i'm just dying to know. gentlemen's bet we will see b3 sometime in the spring, or next fall.
Book 3 energy. Spirits react to a human's energy. So now that spirits and humans will live together most likely the humans inner energy will set them off. This is kinda foreshadowed when Korra found her inner spirit and went Godzilla. Then she energy bended Unalaq. I think their is more to energy bending then we know. Like how Aang used it to restore the environment in the final episode of ATLA
Though I'd think this is a great point for the series to stop since it actually feels now like the series ending without loose ends that seem to be forgoten, I see where they are planning to take the next season, they can easily come up with something cool to follow up considering the unition of the world though I really don't think it will look well after a finale that felt so huge, there really is no bigger deriction they can go so the solution is deeper, I just hope they succed in doing that, as for the names I'd love an energy book but I really don't know what they can name it.
Book 4... Flames under heaven and the war chariots of Zhou!! y'know... becaue its on the Fire Nation and THAT's the stuff we'll get to see there...
Well but since Bryke's not in a good mood for awesomeness lately, I think simpler stuff as "flames" could be enough... "balance" might work fine as AvatarReiko said, if the world is attacked by spirits again, and the Avatar needs to keep balance.