While there is no 3rd Ed expansion available for it, Waterdeep in the Forgotten Realms was an extremely well detailed city. I have had a campaign running in it for many years and there is still so much my PCs have not even had a chance to experience. Probably even more detailed than the city of Greyhawk actually.
While these are all 2nd ed suppliments, there actual game mechanicals are minimal and easily converted. Recommended sources for a really detailed game are the following (can buy them as ESDs from
www.svgames.com if you can't find a hard copy anywhere (in order of necessity):
* City of Splendors - Revised Boxed Set (1st - 2nd Ed)
* Volo's Guide to Waterdeep - a must for detailed games
* Ruins of Undermountain - the mega dungeon below the city
* City System - Original Boxed Set
* Skullport - an underground lawless city in undermountain
Optional Extras:
* Waterdeep and the North - has some info not in boxed sets
* Ruins of Undermountain II - the deeper levels
* Waterdeep Trail Map - Maps of the surrounding area
Modules 2ed:
* (Lvl 7-9) UM1 - The Lost Level
* (Lvl 7-10) UM2 - Maddgoths Castle
* (Lvl 10-14) UM3 - Stardock
* (Lvl 10-12) FRE3 - Waterdeep
3rd Edition Books:
* Forgotten Realms Campaign Setting 3e
* Lords of Darkness 3e
* Faiths and Pantheons 3e
I would very highly recommend Waterdeep, plus you have the rest of Faerun to explore...