This is a few weeks old, but somehow everyone missed it until Kelly messaged me earlier on.

Alyson Stoner

Some voice actor news for Korra. Alyson Stoner, best known for voicing Isabella in Phineas and Pherb or some of her on screen work will voice a character named Opal in Korra. Now it is not confirmed for sure if it is for Book 2, but the source, her own website does mention 2013 and more than likely Book 2 will be all the Korra we get in 2013. That said it could mean that she is just recording her lines in 2013 which would mean Book 3 or 4. Here is the exact quote from her site,
Voicing “Opal” in Nickelodeon’s The Legend of Korra
It is under her projects for 2013 along with some other stuff. So it is again difficult to tell if this means that she will be recording this year or if she will be heard in the show this year. As far as who Opal may be, if it is a book 2 character then most likely it will be Desna's twin sister, AKA Korra's cousin, AKA Unalaq's daughter. This is just complete speculation.