http://ie.ign.com/articles/2012/07/27/the-legend-of-korra-cast-and-creators-talk-book-two?page=1
Here is the article from IGN where they interview Mike and Bryan and some of the cast about what is to come in Book 2.
I will point out the main things using the magic things that are BULLET POINTS
- Mako has joined the Republic City Police Force as a cop, which suggests that the police has opened to not just metalbenders, he is making a name for himself busting the triads.
- Hiroshi is in prison
- Mike and Bryan wanted to get away from Korra training as that was explored in ATLA, and focus more on Korra adapting to her role as a fully realised Avatar. They describe her as a "bull in a china shop" at present, she is going to have to learn how to really respect her abilities and role I think is what they are getting at.
- We may see more appearances from Avatar Aang
- Mike says "We have the most flashback-y of flashbacks coming up" Bryan then hints that it will be deep into the mythology of Avatar and the legacy of the Avatar
- Book 2 will have more romance
- Mako and Korra are together, but with both of them in important positions their relationship can get a bit complicated
- Bolin is going to have a bizzare love story
