Cersei's future is looking pretty bleak on Game of Thrones. She's been counting on having a trial by combat to get her out of her sticky situation with the Faith, but now King Tommen has outlawed them. However, one of Cersei's few helpers, Qyburn, mentioned some "rumor" that might tip things to Cersei's side. What is the rumor on Game of Thrones?
A trial by combat was all Cersei had going for her; with the Mountain as her champion, there was no way she couldn't win. This is likely why the High Sparrow convinced Tommen to do away with the entire process – fans didn't see it happen on screen, but there's no denying Tommen is under the High Sparrow's sway. And the High Sparrow doesn't want Cersei weaseling out of her trial. But he doesn't know Cersei as well as fans do – she always has a trick up her sleeve.
Qyburn is the creepy guy who brought the Mountain back to his current zombie state for Cersei's protection. He also took over Varys' "little birds," the network of children who sneak around Westeros to gather information, so it's fair to say he's a powerful person for Cersei to have on her side. But will he help her with her trial?
The predominant theory is that the rumor Qyburn had the little birds investigating involved the Mad King Aerys, Daenerys' father. Aerys, true to his Targaryen name, was fond of setting just about anything and anyone on fire. Without dragons of his own, he resorted to other methods, including wildfire, the synthetic green flames that are near unstoppable and that Tyrion used to great effect in battles past. Aerys had caches of wildfire stashed away all over King's Landing, which he was planning on burning to the ground before it was sacked and taken over by Robert Baratheon.
If Qyburn was investigating the validity of those secret stores of wildfire and it turned out they really were hidden all over the city, that could give Cersei a secret weapon to use against her enemies – a really destructive one at that, and everyone knows how much Cersei loves violence. Jaime did mention in this very episode that Cersei would "burn cities to ash" for her children if she had to.
Bran has had visions of wildfire being ignited in King's Landing, which seems like a fair bit of foreshadowing for things to come. If Cersei is pushed into a corner, I wouldn't put it past her to burn it all down.