I've always thought it strange that at normal zoom levels I can see partly underground, but I can't see the ceiling, especially since I can decorate my ceiling, but can't do anything with the underground. I know a lot of people are clamoring for balconies (I'm one of them), but I also want basements and caves. Since I like a sunny style yard, I'd love to be able to excavate a little underground area in the backyard for the light-shy egg trees, fireflies, jellisac and such, with the option of sunlamps for those that want to do it the other way around.
My current house has the still, the wood machine and the metal machine in the yard, breathing fuel fumes on my delicate trees, and getting run over by the chickens. The fuel maker is stuck inside, in the corner of the living room, where it clashes with the rest of the decor. With the current options, I can make a /home with a nice living area on the main floor, and stick all the machines and storage on the uppermost level. However, isn't that the perfect scenario for a basement? I'd decorate it Rookist or mad scientist style and hide my extra alchemy equipment down there.
So please, let's make use of all that neglected lower level space and give us the ability to excavate and build underground.