Rick Riordan couldn't stay away from the world of Greek demigods very long. March 2016, the Trials of Apollo Book 1, The Hidden Oracle, is due to be released. Apollo is punished by Zeus and forced to wander the streets of New York as a teenager with no powers and no sun chariot. Supposedly, each chapter will start with one of Apollo's famous haikus. We will hear/see our favorite demigods, but they won't be the main characters, and supposedly won't play a significant role.
I think the Hidden Oracle will finally tell us what happened to the Oracle of Delphi that went silent in Blood of Olympus.