The destruction of Kings Landing has already been referred to once in the series, with Jaime Lannister killing King Aerys II Targaryen to prevent him from ordering the detonation of wildfire caches spread throughout the city. While it is often considered that these were used to defend the city against Stannis Baratheons fleet, that was actually new wildfire created on the orders of first Joffrey and then Tyrion. This means that there are stockpiles of wildfire still hidden throughout Kings Landing. It is certainly likely that the reader/viewer would not have been alerted to their presence if there was no intention by George R.R. Martin to use them at some point in the future. All it would take is for some massive heat source with a large area of effect to hit the city say for instance the flames of a rather large dragon to detonate those caches and destroy the city utterly. Given that any assault by Daenerys and her dragons would eventually result in a last line of defence around the city, it is entirely probable that they will unintentionally destroy the city and the majority of the army defending it. Or it could be that Cersei, in her eventual madness, finds out about these caches and is plotting to destroy the city in revenge during Daenerys inevitable attack, resulting in Jaime killing her to save the city once again.
