hidden, underground caches of Wildfire that the Mad King had planted throughout King's Landing and that Cersei will burn the city to the ground before submitting to the judgement of the Faith. We did see Wildfire igniting in Bran's recent vision download.
I believe that there may be a cache underneath the Sept of Baelor that she will wish to ignite to incinerate the High Sparrow and as many of his fanatics as she can. And I think the plan will go awry and accidentally kill Tommen who happens to be visiting the Sept, or the wildfire will cause a chain-reaction that sets other connected, hidden caches ablaze, possibly under parts of the Red Keep.
I just have a feeling that Tommen will die this season (a King dies every season: Robert 1, Renly 2, Robb 3, Joffrey 4, Stannis and Mance 5) and it would be tragically-poetic that Cersei be responsible for this one - her last child, her children being the only thing she's ever loved, cementing her final, bitter downfall and the fulfillment of the witch's prophecy from her youth.