It bothers me when I mine out a section of a mountain for the steel and then it just has a giant gaping hole in it. I would love if I could fill in those holes.
I know that this could be a balance issue where the rock walls are too strong to build and people would just use it as perimeter fences... But I think it could be balanced if you make them require a LOT of resources to build, or you just make them look like rock and call them "replica marble rock" or something, where they have all the same properties as normal rock, but have 5 hp or something.
You can build stone walls to block off the open areas.
That 'replica' should be do-able. You can make walls look as if they're made from something else in RL, so why not in-game?
EDIT: Something like this?
dud link
EDIT2: Here:
https://ludeon.com/forums/index.php?topic=25834.0
PERFECT!