Plan to Retcon your Worldbuilding

A retcon (short for recontextualize) is an action where an author has to change the fabric of a story with new information, which, if it had existed in earlier stories, would have changed things.  

It’s important to leave room for retcon in your worldbuilding.  If you’re working for novels or for a fantasy RPG campaign series, events may take place over multiple real-world years which require you to make a change here and there.  It’s great if you plan ahead a little and leave room narratively and otherwise for these changes to ‘fit’ with your worldbuilding.

Leave Room on the Map:  Most worldbuilding has a nice map somewhere in the mix.  You’ve got to be able to show the reader where all these cool places are relative to one another.  However, when filling out your map, it’s natural to make every part of it something amazing!  And this is great, until four years later when the new version of your rules set drops and you’ve got three new fantasy races to blend into the world. It’s a good idea to leave a few areas with sparse to no detail, just in case you suddenly need room for a whole kingdom nobody had really noticed before. 
Leave Room in the History: I always find myself not leaving enough room between significant dates for anything else to happen; trying to retcon in a few extra historical events can quickly leave your history feeling like everything is trying to happen at once. A little breathing room or quiet decades isn’t a bad thing.

Leave Known Unknowns: It’s a good idea to have places on the map or periods in history that are known to the current peoples of your world as being poorly understood or mostly lost; either unexplored, thought destroyed, too remote to visit, or forbidden for some reason. These known unknowns can become a great place to suddenly discover new content you didn’t know you needed.

Wiggle room gets future you out of creative jams. Don’t be afraid to leave some.