New map: Hollow World
With all the work I am putting into hollowing out the entire map, I think it'd be pretty cool to actually have a new map mode where the entire map is already cleared out. All the resources you would have gained from hollowing out the entire world would be stored in chests all around the very bottom of the map.
The challenge would be in using those resources to gain access to all the nodes and facilities around the map, building skybridges and the like, finding your way around with no landmarks to tell you which way to go to find said facility, scannable, or ore node.
It would mean rearranging the facilities so Lima is closer to the bottom so players can get access to early game tech, but rearranging things would add to the challenge.