The default setting for The Fantasy Trip: In The Labyrinth RPG is the gigantic world of Cidri. Gamesmasters were only given about a dozen paragraphs of background to the world, but in those few sentences we learn that Cidri was built by an advanced race of humans - the Mnoren - with the ability to travel, unaided, to alternate worlds in other dimensions.
In all they discovered 371 alternate Earths and then used all their accumulated knowledge to build Cidri, a world of 48 continents for their sport and entertainment.
Then over the centuries the immortal Mnoren retreated from view and are now regarded as either bogeymen or distant gods by the many races they transplanted to their "new Earth".
So, who were they? I'm thinking a mixture of the ancient Mesopotamian Anunnaki - so favoured by the 'experts' on Ancient Aliens - and Jack Kirby's Fourth World pantheons of New Genesis and Apokolips (key players in DC Comics until the current iteration of the DC Universe).
Although our campaign has shifted from TFT to Heroes & Other Worlds, we are still using Cidri as our campaign setting.
I can see great potential in exploring the backstory - and fate - of the immortal Mnoren. I just hope our game runs long enough for me to start bringing elements of the world's "creation story" (and Ancient Aliens!) into the game mix