One of the challenges of an MMO is that the game world stays relatively static. You can quickly learn the layout of an adventuring zone (including dungeons) and know exactly what to expect. While there are some benefits to this, it can also be a bit boring. But what if a zone can change?
While you may not necessarily want to change the actual layout of a zone, a mountainous landform should still be a mountain; you can change aspects of it. Perhaps adventurers may run across roaming parties of bandits in the daytime, but those bandits disappear in the evening and are replaced with ghosts and skeletons.
You can take this idea a step further. A dungeon could be overrun with goblins, orcs, and cave trolls. But once the dungeon has been cleared and the main boss(es) defeated, other subterranean creatures have turned it into their lair instead of new (or the same) goblins taking over. It could now be filled with spiders, bats, Troglodytes, and Yaoguai, or perhaps it has been taken over by the undead and living dead such as zombies, skeletons, and vampires. While the general layout of the dungeon hasn’t changed, the decor, the threats, and the loot are now different.
The world can also react in other ways. Using a simple leveling system as an example, areas of a zone could be geared towards specific levels. The forest could be filled with Orc Scouts that are roughly level 8-12, the fort might have more Orcs within and they are levels 10-15, and the main keep is geared towards levels 15 through 20, with the final boss being level 25. The overall macro and micro zones within it have certain expectations of the number of adventurers within it and their levels. But, perhaps there is an abnormally high amount of adventurers within it. Instead of tweaking the respawn rate, you adjust how many orc parties there are. Of course, you also want to have certain thresholds on this. Too many NPCs can be taxing to both the player’s client and the server’s performance, as well as it might cause an unneeded influx of resources (gold, weapons, etc.). But you can adjust these thresholds to keep it exciting based on the number of players in the area.
Using the above scenario, you can also make other adjustments. Perhaps a high-level player is there, twinking others, and you can make adjustments based on that. Maybe the Orcs send out higher-level champions from the fort that travel directly to the disturbance reported to them. Another option is the Orcs have sent out the sound for alarm, and those in the forest have decreased, but those within the fort have increased, making it so that farming out in the wilds has lower returns but giving players a more significant reason to lay siege to the fort.
There are multiple ways to change a single area to keep it a bit more fresh, especially if you add a little RNG to it. And these give a new reason for players to revisit areas that, in many games, they may leave behind once they reach certain levels or thresholds of experience. But any and all of these changes to the adventuring zones have the same end goal; to make areas less static and more fun and enjoyable (which is why we play games).