We could probably also have some simple before the cataclysm “game modes”. Proven that they took place in walled off sections and had few NPCs.
As an example; you could have a scenario in which you spawned as member of a lab security team who is given the job of shutting down an out of control portal generator. It could be a simple as: a completely sealed and well designed laboratory layout, a pair of friendly npc followers to help you survive(food), a bunch of victim scientist NPCs and some scary and difficult monsters (xenomorph style maybe?).
The outcome of such scenarios could also affect the world generation of the main game mode. Winning could spawn the lab as a powerful Old guard stronghold, while failing would spawn the lab as one of those nether influenced areas that have been discussed in the past