With the quest system, something that might be interesting is "temporary biomes". Which is to say, a quest would have you go to a location that doesn't exist, and it gets "painted" into the world like meteors and hardmode biomes do when the quest starts at a location meeting certain criteria. For instance, an island might appear in the sky, or an open space in the ocean. An existing cave might get converted into a special variety, or a small dungeon might get carved out of solid rock. You go there to complete the quest, and when it's done and turned in, as soon as no players are nearby a similar process makes the biome go away. To prevent people losing things in these areas, it might be necessary to apply the anti-build debuff that the Old One's Army event adds while in the temporary biomes, though.
