Hi,
One thing that I think would be cool in the base layout screen would be the option to lay a corridor piece. It should serve no obvious purpose other than to form a corridor.
I'm no dev, but I can't really see this being a difficult thing to implement.
In Game Benefits:
Allows you more options to design a defensible base IE choke points etc
Bases can be more customisable/unique to the player
In Game balancing factors:
Monetary cost to dig out tunnel
Less space to build rooms which have an obvious use
Cheers
Jimmi