Is there a way to export a specific city or island into a new scene and modify/export the terrain/mesh data? or export a heightmap from Cesium?
Currently, there is no way to export specific areas of terrain or other Cesium ion tilesets. It’s possible we may add a feature to get a heightmap from terrain in the future. You can check out that issue on our Github repository here: Convert subset of terrain into Unreal landscape · Issue #351 · CesiumGS/cesium-unreal · GitHub
If you have any other information or suggestions regarding this, please feel free to reply here or add them directly to the issue!