I think most kids make up imaginary worlds, I just never stopped.
Some examples in rough chronological order, starting from the earliest one I can remember in late elementary school
1: The adventures of a superhero called Crystal Man (likely yoinked from the robot master of the same name from MM5) who could manipulate light. His arch enemy was the septic tank monster.
2: This is when I started writing down some lore. A race of little gnome-like critters that were obsessed with sharp objects and fire, and couldn't make their own blood so they had to steal the blood of others, which they called "ketchup". These guys had a whole religion revolving around their progenitor. They reproduced by division like bacteria and all descend from a single individual who they worship as their god. Their home planet is made of Styrofoam. They also had an open circulatory system like a mollusk. I was a weirdo what can I say.
3: A sometimes-fantasy sometimes-sort-of sci-fi setting that exists on the inner surface of a sphere. At the center of the sphere is a star-like light source that is either a god of order or a servant of such a god. Surrounding the sphere, underground from the perspective of the inhabitants on the surface, is a writhing mass of viscera that is a god of entropy and chaos. The world exists either near the beginning of a new universe or near its end, it's not sure to the inhabitants which is true. Magic is possible because the laws of physics are either still sorting themselves out or because they're breaking down. The light is either trying to banish the chaos and establish order, or fighting a losing battle against the incarnation of heat death.
4: Humanity has figured out FTL travel, but it requires brain uploading, extensive gene modification, or other cyberpunk-esque shenanigans. Earth is left to a tiny population of people who were either too poor to afford these augmentations or had ethical/religious objections to them. Thousands of years pass, and the remnants of Earth civilization have flourished and there are several large nations divided largely along sectarian lines. They figure out another method of FTL travel that doesn't require those modifications, and strike out into the stars to find the rest of humanity. None of the earlier group of transhumans remains, only non-sentient zombies, the remnants of genetically modified humans who painted themselves into an evolutionary corner, as well as the maybe sentient but probably not copies of human brains residing in giant supercomputers. These remnants are called "elves" by baseline humanity, and discovering the myriad ways the various colonies of elves did themselves in drives the narrative.
5 My current project: There's only one other sapient species in the galaxy. This species achieves sapience around the same time as humanity (somewhat arbitrarily determined in the setting to be when we started burying our dead around 100 thousand years ago). Unlike humans, this species evolves a written language from the get-go out of a scent marking behavior, so they have a documented history that stretches back to the dawn of their species. Very early on in their history, a religion emerges whose central tenant is that there are other sophonts dwelling among the stars, and they have been given a divine commandment to seek out these sophonts. The religion is like if the Catholic Church, the Vestal Virgins, Bell Labs, NASA, and OceanGate merged. Most technological progress is driven by their research monks trying to find out ways to fulfill this commandment.
100 millennia later and despite their interstellar missionary efforts they haven't encountered so much as a microbe, and their society has largely secularized as a result. The faith is a shadow of its former self, but still keeps sending out missions in the increasingly futile hope they'll find someone. Then one day, they find these weird bipedal flat-faced hairless apes.
These are all paracosms dreamt up for their own sake, playgrounds to get lost in while daydreaming, though I've actually attempted some stories set in my most recent setting, and have a few constructed languages for those alien critters.
What about you?
