HBO cast two new characters for season eight. There's not a lot to go off of at the moment, but we do know that both of the casting notices are in search of young actors. The first is for a "Northern Girl" who's between the ages of 8-10 and comes from a good family full of soldiers. The notice further describes her as "a straight-talking Northern-accented girl, someone remarkable for her dauntlessness and integrity."
The second role is for a boy of similar age to the female part, with a "physically fit and agile look" and a striking face. The character will be from a poor family, and the actor chosen must be able to "completely own the scenes that he appears in" (which hints that this isn't merely a cameo).