This is something I've been meaning to ask.
When I heard about being able to raise and lower the ground I instantly thought of WURM and if it was possible for Xsyon to have the mining like they do.
Personally it won't bother me at all if it isn't in as it sounds crazy as hell to implement in any game that isn't WURM.
This would be amazing especially when combined with mining for resources. Currently no other mmo has attempted mining in such a fashion. As for underground cities, I believe it's improbable as it leans more towards a fantasy take than a sci-fi/post apoc setting that borders on realism. However what is plausible in my opinion is building a village within or on a mountain side/rock face as there is more than enough support to prevent the collapse of the structure.