Here's a list of suspects:
The good guys (Tyrion or Sansa or Shae)
The bad guys (Tywin, Cersei, or other Lannisters)
The other guys (Olenna, Margaery or other Tyrells)
The new guys (Oberyn and Ellaria)
The magical guys (Stannis and Melisandre and their plan to kill the usurpers)
The sneaky guys (Dontos, or Littlefinger…