During pre-alpha I examined the idea of doing stuff in between floors, but decided against it for a number of reasons, although later during alpha we did eventually get the "non-map maps" that are Waste and Garrison interiors, the latter of which can and do actually send you to other maps, kind of acting as intermap links.
I do actually have plans for other smaller/tiny maps along the lines of what you're thinking here, thematically, but this is post-1.0 stuff (they've been on my TODO list for a good couple years now, just can't add them yet because they're too low priority). They would not, however, be located in between 0b10 floors, since the aim of the complex design is to have an essentially predictable path (it's the same reason you don't see prefabs in Factory). This would be something you could find in branches, especially caves.
Can't give any more details though since it's not a guaranteed feature--depends entirely on whether or not I have enough funding to develop it down the line
