Interior Walls
-They could be slightly cheaper, build faster, with less HP.
-Increase happiness with a room, over the thick, cold metal walls. These interior walls have decorative paneling!
(Apologies if this is elsewhere, but most of what I see about walls/structures is for modifying combat, not decreasing resource use or upping happiness.)
Selling research, either so you have to do it again or just for quick cash, to a passing colony ship.
(Buying was suggested. I don't think selling was. This also might already be planned with factions/trading research)
An event where a weapon breaks or is lost.
A training dummy to shoot/punch to upgrade your combat skills, outside of combat.
A flare gun that scares off animals or recalls colonists. (has a chance to catch something on fire)
Smaller/1x1 dumping areas, to allow more flexibility. (Plus, you can make an S.O.S. with rocks!)
A recreation/social room or lounge chair, where idle citizens go to increase happiness. (Could also be used as a focal point, if the game gets an alarm to call citizens back to base, during an attack.)
A backpack/cart that causes your colonist to go slower, but carry more.