Roofs that hold water: reduce heating and cooling costs, increase life of roof, and create a rooftop garden! Would be great on a shipping container home, as there might be a risk of collapsed roof otherwise if drainage isn't sufficient.
secret door to an underground storm shelter/panic room/hid out in the kitchen island! Best secret passage ever! Definitely a dream home feature! (would also be good if someone broke into your house and you had to hide somewhere) Zombie apocalypse.