Just guessing, but some of you out there might be thinking of starting a new campaign and are thinking that you want to build a new campaign world for it. But going to all that trouble for one campaign isn’t the idea - This is going to be a world you can continue to use. But you’re creating a new world, so there must be something wrong with the old one (assuming you have an old one). Is it that the players know too much? That can be the end of a campaign world!
So here’s the plan: You’re going to write a campaign (OK at this point you’re going to outline a campaign) where the party will eventually do something that will lead to a major change in the world. They may defeat an evil empire that is currently controlling the continent or return a princess to her father’s throne or find a lost city that returns to its former glory - something literally world changing. This might be the reason for the new campaign world in the first place - you didn’t want to mess up the current one.
OK - so you do all the smart world building things. You start small and keep expanding. You figure out one city and surrounding region and as the campaign grows in complexity, you keep developing more and more regions as they are needed. You introduce the main goal early on, but without the party knowing that it is the main goal. For instance, they start working against the evil empire, never dreaming that they will eventually work to topple it. The campaign grows and the party gets powerful and eventually they destroy the evil empire and all the lands are free.
Great - time to start the next campaign. Leave that one finished as they have certainly gone out with a bang, and they served the purpose. All those adventures allowed you to build the world as you went, taking digestible sized bites as things progressed. But the new campaign has two new players in it, and you don’t want the established players having more understanding that the first campaign carry forwards. It’s simple! The second campaign starts in the same world, but 50 years in the future.
50 years in the future, just about everyone from the first campaign would now be dead (unless they had some magical youth stuff). The evil empire is gone, so the politics and even the borders have changed. Everything has changed! As GM, you still have the same maps, the same cultures (slightly altered), the same races and monsters dwelling in generally the same places. But to the characters, everything is different. The shops are all changed around, the people are different. While it may be the same noble families, there are different people. The change that the first campaign accomplished is remembered, probably with a holiday, but probably not remembered the way the players remember it. That will be part of the fun - Changing things. History may be written by the victors, but it’s always written with a bias. Wouldn’t it be great if one of the original party’s enemies or rivals turned out to be the writer of the main history book? Anyway, things are different now, and the party has the fun of learning everything over again.
In some ways, this is how Fletnern began, but in our world, it was because the first campaign failed at what they were supposed to do. It’s less that they failed and more that the group who was playing broke apart and was replaced by a different group, with very few hold overs. To prevent the first group’s failures from affecting the new group of players, I walked the world forward 25 years. But 25 is a tough number, because some of those people are still alive, and it’s difficult to balance the same NPCs 25 years later. 50 works better! Give it a try.