I just read Vincent Baker's Apocalypse World ^RPGGeek, and like all of his games it contains more than one brilliant thing. But for this case, the brilliant thing I'm going to suggest that you take a look at is the Countdown Clocks he created. Without directly stealing the clocks (which I intend to do for my own games, even outside Apocalypse World), you could take the major actors in the world (rain of fire, new dungeon) and create progressions for what will happen if the PCs don't intervene, and / or what might make them move along the progressions.

Just an off-the-top of my head example for "rains of fire":

1. Rain of Fire appears in the sky
2. If PCs do nothing, villages begin to burn
3. If PCs do nothing or fail, the richest forest in the land is set on fire
4. If PCs do nothing or fail, the forest is consumed and the world is blanketed in darkness and ash
5. If the PCs do nothing or fail, the kingdom is plunged into n years without summer and the frost giants move down from the utmost North...

This isn't exactly how the clocks work (his have much more awesome ladled over them), but it puts a flashing arrow into the player's picture of the world saying "BIG PLOT HERE!" and if they choose to ignore it, or suck at helping, you know where the consequences are headed.

Also - if your "giant dungeon rediscovered" is progressing on "If the PCs do nothing..." then you've presented an interesting dilemma - which part of the world do you want to save?

What if one PC has peasant roots and his ancestral village will be destroyed by the fires and one has royal roots and his throne-to-be is will be destroyed by what's crawling out of the dungeon? Or what if they PCs are strangers and just have to choose between saving the peasants and saving the king?

I just read Vincent Baker's Apocalypse World ^RPGGeek, and like all of his games it contains more than one brilliant thing. But for this case, the brilliant thing I'm going to suggest that you take a look at is the Countdown Clocks he created. Without directly stealing the clocks (which I intend to do for my own games, even outside Apocalypse World), you could take the major actors in the world (rain of fire, new dungeon) and create progressions for what will happen if the PCs don't intervene, and / or what might make them move along the progressions.

Just an off-the-top of my head example for "rains of fire":

1. Rain of Fire appears in the sky
2. If PCs do nothing, villages begin to burn
3. If PCs do nothing or fail, the richest forest in the land is set on fire
4. If PCs do nothing or fail, the forest is consumed and the world is blanketed in darkness and ash
5. If the PCs do nothing or fail, the kingdom is plunged into n years without summer and the frost giants move down from the utmost North...

This isn't exactly how the clocks work (his have much more awesome ladled over them), but it puts a flashing arrow into the player's picture of the world saying "BIG PLOT HERE!" and if they choose to ignore it, or suck at helping, you know where the consequences are headed.

Also - if your "giant dungeon rediscovered" is progressing on "If the PCs do nothing..." then you've presented an interesting dilemma - which part of the world do you want to save?

What if one PC has peasant roots and his ancestral village will be destroyed by the fires and one has royal roots and his throne-to-be is what will be destroyed by what's crawling out of the dungeon? Or what if they PCs are strangers and just have to choose between saving the peasants and saving the king?