About 2 days ago, Seychelle Gabriel, the voice actor of Asami, tweeted that she had just read the first script of BOOK 3 of The Legend of Korra and that she's loving Tenzin's character.
Unfortunately, the actual tweet was deleted, but I managed to get a Screen Shot taken before it was deleted. If you're interest in seeing the full tweet, you can see it on this post.
Anyways, it looks like we can take this as conformation that Book 3's plot line and overall idea is finally complete. I expect a few minor revisions soon, but nothing major in terms of plot changes.