It should always be plot dependent. At 15th level there should be massive numbers of hooks or long-forgotten incidents that have snowballed into much larger events.
Here's what my players did IMC. The backstory is that the players were following what they thought was a child-murderer and managed to screw up an entire demi-plane, releasing a massive horde of undead on the world (So very much *NOT* my original plan).
1. The players are approached by an elven ally. It seems that the undead horde includes a few individuals who would rather not be involved, in particular a lich that's a distant relative. The lich is willing to sit out and will even provide some magic items to the living if they provide him someplace to call home while he's waiting for the demiplane to settle down, preferrably an island. The ally wants the heroes to negotiate on the lich's behalf with the King.
2. Undead invasion begins- Corpse Gatherers assault the royal city, some tunnel in from beneath. Not quite the assault on Gondor but close enough for my players.
3. Heroes are contacted about an ancient fortress that is rumored to have an army in stasis, an army possibly powerful enough to defeat the undead. The players venture to the arctic circle and fight their way inside, only to find the army consists of mutants that were considered uncontrollable by their creators. (They start lawful evil but quickly become chaotic, sometimes to chaotic good but sometimes not)
4. The cleric is given a quest by his god to petition a self-exiled demigod to put the demi-plane back together. They fight their way past the defenses of the last standing temple and have to convince the demigod, his wife, children, etc, that he should get back into the heroing business via (gasp) roleplaying.
5. The bard hears about a distant, almost fabled elven land that's been taken over by some of the liches and vampires they set free. Feeling responsible, he and the others travel far across the world to investigate, using their contacts to get hired on as caravan guards. (*COMPLETELY* the party's idea, I just wanted them to realize how big an impact their actions have had) Things are worse than they expected since the elves had long been conquered by a dragon necromancer and are too broken in spirit to care if the boss at the top has changed.
I admit, I had a massive hook from the war they started but most of this would've happened even if it wasn't the party's fault. Look at your own game, find places they left loose ends and try to think of the worst *plausible* result and run with it. The players may never realize they were the ones responsible but they probably will and it will help cement the causality of your world.
