I think I would be more helpful if I knew what you mean by subject. Do you mean like Mountains? Lodestones? Firefighters? Starships? And explanation of what? You said world building, which is the subject of what I talking about, but if you are looking for one specific piece of what it takes to make a world, or things that should be found in a world, I would go with homes. Residences. Since a story is nothing without it's characters, establishing where they rest their head is a good subject to address :P

But still, I have no idea what you mean by subject, and I must be passing out now, so if my mad ramblings of a tired person are found useless, I apologies.