Well Korra was only supposed to be these 12 episodes, then Nick ordered 14 more for Book 2. So they made another self-contained arc that will be Book 2 (which is currently being animated iirc). Brian and Mike did say they had ideas for what'll happen to the group after Book 2 but were waiting for the okay to put them into motion (they were prepared for the possibility of those stories never being told).
I am glad that Korra's gonna total 52 episodes; that almost rivals the original series's 61. I hope this also gives more room for character-centric episodes for Bolin and stuff, as we had them for Sokka and Appa and stuff in A:TLA.
