Fortunately this would be 100% studio, so no bedroom so worry about.
Slope and gear wouldn't be an issue as there is room for a driveway and car space right down to the entrance. But certainly a good thing to keep in mind. Lots of gear would be going in and out all the time so it needs to have easy access or people wouldn't come back!
As for that underground town!! I have heard of the name Coober Pedy lots of times but I never realised it was an underground town!! Very cool. I would drive there for a visit, but it's nearly 2000 kms away... For anyone interested -
http://www.hotelclub.com/blog/the-underground-town-of-coober-pedy/ Spacey, I'm a mechanical engineer though so less knowledgeable in construction! I certainly did think about water runnoff and underground water flow during rain. It would certainly require a bit of thinking for adequate water diversion. It's right next to a fairly busy road for trucks, so fortunately not as bad as trains, but still noisy. It's a fair bit lower than the road, so I'd need to check line of site though and ensure it's well out of the way. I think there is also open pit mining somewhere in the area! But it's well far enough away. Only issue is the dust.
If it needs it, I'd actually consider making floating iso rooms. It would at the very least be a lot of fun! I like the idea of making a main room with 2-3 iso rooms and a control room, all independently floating, but you're starting to talk serious $$ when you get into that, and some serious room is required too. Floating iso rooms would be great for bands which want a multi tracked sound, but really need to see the other band members in action when they play. You can always dream! And hopefully bring to reality.
Further still, I'd love to design something which could be expanded into 5.1 mixing and video capabilities. I'm starting to get into video these days so I may get heavily into that field too!
And finally, I'm 25 and have so many dreams... Still a young spring chicken, so I guess I'll have to wait and see what life has in store for me. Fingers crossed I get to build my dream!