They could add new massive underground “open interior” where players can build, accessed from caves in Exiled Lands similar to Blackreach or Siofra River.
It would kind of be within the same map, but would also be able to unload the overworld partially, without separating players even further with a completely new map.
Would be a nice spot to find new materials, treasures, architecture and creatures, and new fancy bazaar items.
Would also pose new challenges for players, getting the supplies from the outside, choosing new good spots close to entrances but well defended, etc.
Also, hanging cave ceiling bases.