Basically, would be cool if cellars were all on the same layer with mines and caves. And in them you could even build a mine to get to the next layer. Would add a new level of depth to the game!
I would be so pissed if some guy broke into my house because he tunneled into my cellar...
Besides, wouldn't this require an overhaul of building interiors? As far as I know, each cabin is it's own self-contained area, only linked to the map through the door. In other words, it's not part of the map. I suppose I wouldn't really know, though.