When I was working as "VIP" staff on UN I was going to make myself a house. Except my house was going to be a complex "Adventure-style" quest.
I had already planned out all the mechanics and started writing out the backstory of the mansion as well as the overall plot, and started making concepts for various NPC's in the house. The goal was going to be to go around and collect items and solve the mystery of Dusty's Manor, which revolved a lot around avoiding shadows and getting the lights back on(hence the dark areas on the map). It was going to have its own inventory system and everything, but oh well.