I've reached the point where I now have more spaces dug out in the Hero Hideout than I can actually use, since I've already built all the allowable rooms. And there are still more spaces that I can clear!
I'm assuming that the game developers are still considering the game balance of addtional functional rooms, but in the meantime I would suggest allowing the construction of 'decorative' rooms that fill an empty space but don't provide any function or bonuses.
Some possibilities: A cave-like space with animated bats flying about
A storage room full of crates, barrels, etc.,
Table tennis - defense bots periodically play against each other
Dungeon - darkened space beyond iron bars with glowing eyes visible