The idea: an underground city with a lot of interesting things. If it works out, there will be a main quest in the mod relating to a character who is aware that the Ministry of Truth is going to destroy Vivec City after the Nerevarine destroys the Heart of Lorkin.
This video basically shows a cavern entrance to the unbuilt city via an undercover shack outside Khuul. Again, it's super unfinished, but I've only started using the Construction Set a week ago.
http://www.youtube.com/watch?feature=player_profilepage&v=gZ5THVVGc1E
The mod will use CLothiers of VVardenfell. MCA, and Complete Gothic Attire to give some sense of something new to the characters.