Lots of GMs want their campaigns to tell an epic, inter-connected story with plot threads woven over dozens of sessions. But as soon as they plan one, their players take a left turn two sessions in and never actually interact with 90% of the material the GM planned.
Here’s the secret: don’t write out what happens. Instead, develop big problems and big antagonists.
In other words, create an antagonist who has big plans, like raising an undead army, leading a military coup, or killing a god. Make it something that will upend the world. You can even create several antagonists like this.
Then place your antagonist most of the way towards his or her goal. Give him or her resources. The army is half-built, the country is destabilized, or the antagonist is only a few pieces away from assembling the Staff of God Killing.
You’re now ready to start the game, during which, you must make these situations visible and evident in the PCs’ every-day lives. Every town is attacked by undead, everybody’s talking about the weak leader, or every powerful mentor is scrambling to find adventurers who will find other pieces of the Staff.
As the PCs continue in their story, ensure that every major decision the PCs make for or against these antagonists has an impact. If they find a piece of the Staff, they’re attacked by the antagonist’s agents. If they ignore the undead attacks, those attacks intensify and the shops they rely on for supplies are destroyed.
Create a problem that the PCs ignore at their own peril, so that the game is defined by their actions relative to that problem.