I'm pretty sure I can predict the entire season from the first episode.
Korra no longer wants to be the Avatar. She's gone through absolute hell both physically and psychologically the last few years and she's sick of it, which is why she's trying to hide and forget about the world doing the cage fighting. So the main subplot of the season will be finding Korra again and convincing her to be the Avatar. I'm gonna go out on a limb here and predict that she somehow reconnects with her past lives, because otherwise I can't really think of a compelling reason for her to be willing to be the Avatar again. Nobody but her past lives really understands what she has gone through.
The secondary subplot of the season will be the popularity of Kuvira vs King Wu among the earth kingdom citizens. I assume they have to muddle things up with shades of grey, because there's no real room for black and white in that conflict. Mako will have to thwart either an assassination or kidnapping attempt against King Wu, meanwhile Opal and Kai will discover evidence linking Kuvira to either the attempt against King Wu, the bandits, or both. They'll try to get the evidence to Bolin, but things will be complicated somehow, causing him some sort of moral dilemma instead of it just being an easy choice to turn against Kuvira.
Gonna watch the Book 4 trailer and see if that changes my predictions any.
*edit*
Not really, although the various scenes of people fighting are very suggestive. I guess it's more likely that Toph helps her decide to be the Avatar again rather than reconnecting with her past lives. Unless Korra saying Toph is an enormous tease and Toph isn't actually who she sees in that scene.
