Mike, Bryan Joachim and Ryu Hi Hyun did a signing of the Book 1 Air Art book just yesterday and there was a Q and A session.
While not that much was said, there were some very interesting pieces of news. All of this is based off of reports from people at the event, so every detail may not be fully confirmed, but the main jist is correct.
The main thing would be that as we know from a few weeks ago, Lin is not going to be in Book 2 that much, but will be heavily featured in Book 3. We will learn much more about Lin and also her family as well as Earthbending. So I think Lin's father may be at least revealed.
Korra and Asami's friendship also will not be featured too much in Book 2, but will be shown better further down the line. It is confirmed that the two are good friends.
The Equalist plot will not be coming back, Bryan said that in his mind there are still a few who have meetings under bookshops, but nothing serious.
With regards to background on the Triads they said Republic City Hustle will be the main thing exploring that and not that much at that.
So not too much revealed, but I think by far the biggest thing is the reveal that Lin, her family and earthbending will be explored in Book 3.
Thoughts ?
