Verily, there is no lack of paracosms among the readers of this site. We have:
Rainbow Town, located near Carmel CA, and populated with an impressive cast of characters including Anna Pavlova the ballerina.
A party of tiny interstellar explorers arriving on the earth.
A cat planet with city maps (the capital map being a solid five feet square), history, mythology, cosmology, intrigues involving the canine assassin QVB, and yes, an anthem; also a couple coherent collections of what must amount to religious parody, complete with archaeological and sociological histories of its coming to light, the fates of the tribes that wrote it, etc.
An imaginary boat whose travels around the globe were documented in an extensive seventy-page log, though for the most part it just sailed and refueled.
A hierarchical society based upon the structure of a bookshelf, where the doll at top is the queen, the Star Wars figures on the next tier do her bidding, and the lesser figures (trolls, etc.) occupy themselves with social visits or adventures to the Land of the Bed.
One reader asks "When, at what age, do people *stop* creating paracosms?" and another points out: "Isn't this the concept behind role-playing games and their techy cousins? Certainly one of the best-known and most extensively developed paracosms is, of course, Middle Earth, complete with myths, languages, and all."
Quite so. I suspect that those who retain the paracosmic urge past childhood find themselves quite naturally drawn to fantasy and science fiction, since these genres necessitate the creation of a world. In fact it's rather easy for the world to overwhelm the story; in junior high creative writing classes, I never got around to the science fiction epics I was supposed to be writing, as I was too busy coming up with the history of the next few centuries and the grammar of the alien languages and so forth. There are still notebooks sitting around somewhere with reams of the stuff. The same thing can happen to readers; as evidence there is Tolkien's aggrieved letter to a cartographically inclined fan:
While many like you demand maps, others wish for geological indications rather than places; many want Elvish grammars, phonologies, and specimens; some want metrics and prosodies.... Musicians want tunes, and musical notation; archaeologists want ceramics and metallurgy; botanists want a more accurate description of the mallorn, of elanor, niphredil, alfirin, mallos, and symbelmyne; historians want more details about the social and political structure of Gondor; general enquirers want information about the Wainriders, the Harad, Dwarvish origins, the Dead Men, the Beornings, and the two missing wizards (out of five); in short, they want lore."
I am also directed to the imaginary lands of Angria and Gondal created by the Brontë children.