I was wondering about this subject. I've seen some posts about ship building, but not many about cities and bases.
Personally, I would like to build a replica of the Black Mesa base from Half Life. It might not be very functional, but I think it would be extremely cool.
Another design I would like to try building is the city of Omikron from the game, Omikron: The Nomad Soul. For those of you who have never heard of this game, it came out in 1999 and featured David Bowie. It was made in France just like this game and was just as innovative for it's time. It was set in a vast city built under a dome and had sections with very different architectural styles.
Do you have any kinds of plans for this yet? Are there any designs that you would like to see people build?