Here's what I think could happen. This next episode while Loras and Margery is standing trial, Wildfire kills everyone in the Septon, leaving Cersei in power of Kings Landing. When Cersei learns of Jon and Sansa in Winterfell again, she will start gathering a larger Army. Dany lands in Westeros somewhere closer to the north, maybe the Iron islands. Learns of the battle with the WW at some point, teams up with Winterfell. Cersei learns of Dany and after the battle with the WWs is over she realizes that she's whats standing in Danys way. After suffering a huge lost of people and possibly some dragons to the WWs, it becomes Dany and the Starks against everyone else who hasn't pledged to her. I think Cersei could get a large number of supporters, due to the fact that other families fear Dany because of the way the Mad King ruled. Hugely hypothetical but we will learn a lot more after this coming episode. Side note, I think the secret Cersei was talking about was either the wildfire, Sansa and Jon Snow, or Tyrion with Dany. Last weeks episode made it kind of clear though.