1. It could be Varys on his way
2. It could be they found out where Tyrion is, perhaps even that he wasn't involved in the death of the king, and that he's helping a Targaryen queen that has dragons and she's on her way to forge an unlikely alliance
3. It could be that Riverrun has fallen to Jaime and he's on his way back with an army at his heels
4. It could be that they've found out the Tyrells were involved in poisoning Joffrey and in telling Tommen she'd turn him against Tyrells and start a civil war
5. It could be something from the High Sparrow's past
6. It could be something about what the High Sparrow is planning, that would undermine his power
7. It could be something to do with the White Walkers, and that all able-bodied men must be sent north - including the Sparrows
8. Of course, it could be wildfire after all. I'd just be a bit disappointed as we've already had that once.