While building a bunker-like colony a little while ago, I had a thought: what if there was a world setting that turned the outside world hostile?
Let me specify a little. My plan is that your colonists would arrive the planet decked out in space suits, because the planet has a poisonous atmosphere. Take that suit off for a slow and painful death.
To counteract this hostility, you have to build airlock systems and closed-off doors; air purifiers, oxygen pumps, etc will also be necessary. Leave no open gaps and be very careful during firefights.
I feel as if this option would add a nice factor of extra difficulty for Rimworld, if the player should so choose.
Something similar was in a prototype version of Rimworld called Eclipse Colony/
http://ludeon.com/forums/index.php?topic=4104.msg40386#msg40386
Although I wish you luck if you want to try and mod it, albeit how difficult that will be.