I was about to reïntroduce my Wine/Cognac/Burwitch Brew caves, but I see they have not been forgotten 
I agree with Medierra though that it would be a bit too limited for an entire UG theme set, or rather I think it would be perfectly possible to create one huge location base on this theme, but then it would not make any sense to have any others both in terms of variation and in terms of lore (vintage wine and liquor ought to be rare). That taken into consideration, creating an entire UG theme just for one dungeon would just be inefficient. Unless ofcourse GD has/had a region devoted to wine/liquor production.

In general, I think there are three particularly noteworthy great ideas about UG worlds: Verne’s, H.G. Wells’ artilleryman’s dreams from the War of the Worlds (see image above), and Tolkien’s Moria. The latter does not in any way apply to GD’s setting, so that leaves Verne’s and Wells’. Wells’ thematically would be dead on, but Verne’s mainly consists of flora and primitive fauna. However, it has something in common with Wells’, which I think is precisely what makes these ideas so captivating. Rather than a network of tunnels or caves, they picture a large open domelike underground envirnonment where buildings and/or objects are placed in whole as they would be above ground, though often some of these objects are partially integrated in the subterran surroundings. For a more contemporary example : think Gundam Universal Century Federation HQ at Jaburo. This is what I think distinguishes a well designed UG environment from a truly great one, and personally I would like to see an UG theme in GD based on this concept as well.

I really like the railroads idea in particular too. Moreover, apart from forming an immensely atmospheric UG theme and fitting the game’s setting perfectly (above: an image of the Moscow Metro Komsomolskaya station), it could be used as thé solution for tying in future expansions. In the main game a large abandonned UG station could be “unlocked” which is then used to fast travel to the new areas of future expansions, similar to the Athens-to-Crete ferry in TQ, but perhaps with a nice cutscene this time. An old railway network would lorewise be ideal to connect even several different regions like the main game are, a northern winterland, and a southern dessert. Especially for the winter themed expansion it would provide a magnificent entry. Just think of the Syberia series if you want to get an impression of what I mean.
Alternatively, I had got this crazy idea from a comic about a submerged airship accessed from underground. It might sound a bit odd to keep an airship underground or under the surface waterlevel, but it would be the ideal way to hide it. If it floats on air, it floats on water, so if you tie it down underwater you’d only have to release it for it to immediately surface and airborne, given that the craft’s structure is sturdy enough to resist the waterpressure while submerged.
Below is an extract from the ridiculous yet ridiculously beautiful comic my little brother bought as a toddler, simply because even then he could see that it was ridiculously beautiful. The craft in the comic is actually not an airship at all, but it is huge, and hidden underwater.

Also, I find Mssangols’ pictures from Norway really appealing too. They exhale a grey harsh yet striking beauty that breathes cold, yet the scenery shows no signs of actual frost. It would fit well with GD I think and it reminds me of when I went off-track in the Bré mountains in co. Dublin.

It shows how a railway track can blend in nicely too. The picture is in black and white because the weather was so grey, I actually didn’t notice my crappy phone camera (notice the dead pixel in the middle) was still set on B&W filter. In retrospective it probably was a bad idea going out hiking at all, let alone in my classic outfit wearing flat-sole shoes and woolen cloths 
Finally, let me apologize for this disturbingly chaotic post. I didn’t have time to reply earlier, thoughts piled up, and I figured this thread was for brainstorming anyway 