And here I am imagining what it would be like to first astronauts landing on one of your worlds. What would an explorer feel upon seeing vast cities and machinery, but seeing no trace of the people who built it. Was the city built by a species who all died in a plague 1000 years ago? Is it an abandoned City of the Gods, who were destroyed by another pantheon? Did the inhabitants all leave to a higher plane of existence? Was it built by nanotechnology in anticipation of another alien species' arrival? Add a character who is driven to find out the truth and let them explore.

Or you could always go with the "the world really is empty" existential horror, like in Yume Nikki. As a world with only one character and very few monsters, it still manages to be terrifying.