One of the things that will differentiate a sandbox from a theme park is that the player's actions or lack thereof can create persistent effects. Obviously, if we let every PvE encounter area remain in the game forever, in a very short while the whole map would be covered with exhausted environs and the slaughtered remains of their inhabitants.
Some of the PvE content that a player will encounter will automatically be removed by the game after a certain period of time passes without any player interacting with it. In this way we can constantly present players with unique adventures that are fresh and ready for you to venture forth and earn ever greater fortune and glory.
However, a very few of these areas may morph into something much more permanent. Occasionally, when a player defeats a PvE encounter such as a wandering creature or encampment, instead of being quietly removed by the game system, that area will instead open up for habitation by even more powerful creatures lured in by the scent of fresh meat
An example of this - A group of 3 friends are levelling in an area and stumble upon a bandit camp. They clean out these guys as any law abiding citizen would, but hold on, later that same day the bandit leadership finds out what happens and dispatches a group of hardened gank killers to massacre the scum that dared kill his men. This creates an even greater challenge in this area and extends the content contained there.
In this way, a game area can become unique over time, gaining a story of its own that is formed and shaped by the actions of the players. People will know how certain features of the land were created, and those memories will invest the place with much more value to the world than mere random buildings would ever generate.