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.