I'm almost positive it's about wildfire. It seems like the only thing that has been set up.
1. Bran's vision showed wildfire, not only reminding the audience what it is, but showing that the pyromancers had a stash deep within the city.
2. In Bran's vision, the wildfire exploded, which isn't what happened with the Mad King. That seems to be a vision of the future, not the past.
3. When Dany had her vision, she visited a throne room that looked like it was either snowed in, or burnt to ash. Ever since then I've felt like when Dany finally made it to King's Landing, she would find a destroyed Throne Room.
4. Cersei has said several times that she would burn the city to the ground. She was the one in season 2 that originally had the pyromancers producing wildfire, before Tyrion took over.
5. Jaimie stopped the mad king from burning king's landing, and forever tarnished his name because of it, so it would be especially compelling to see Jaimie react to the news that Cersei succeeded in doing what he lost his honor to stop.
6. Olenna's comment to Cersei. "What do you plan to do? Kill them all?" feels way too much like foreshadowing. How will she kill them all with just Frankenmountain?
7. Cersei asked Qyburn about the rumors. "Rumors or something more?" and he said "More. A lot more." Seems to point to there being a lot more wildfire than what Tyrion used up. His little birds would have been small enough to go through those little tunnels to investigate.
Someone mentioned it could be about the Tyrells killing Joffrey, but who would they have found that out from? Littlefinger and Sansa are thousands of miles away. Ser Dontos is dead. And the only other two that knew are Margery and Olenna. If someone else knows, they sure haven't set it up. And even if it was, what would that matter anyway? Cersei already hates the Tyrells, she's already been plotting to get rid of them, finding out that they were involved wouldn't really alter her motivations enough for it to be a compelling plot point.