Adam Roberts on the global success of Tolkien:
One reason Tolkien’s imaginary realm has proved so successful is precisely its structural non-specificity. What I mean is: Tolkien treats material that has deep roots in, and deep appeal to, various cultural traditions; but he does so in a way—as fictionalised worldbuilding rather than denominated myth—that drains away much of the poisonous nationalist, racist and belligerent associations those traditions have accumulated over the centuries.
This is very similar to what Henry Jenkins has to say about Harry Potter, where he argues (my paraphrasing) there is a world broad and shallow enough to include the potential of every individual reader's inclusion.
I'm struggling to find more examples of these ubiquitous, non-exclusive properties. I think Game of Thrones is hypnotic drama, but it doesn't have the same sense of active escapism: it would be difficult to see yourself in the GoT world (where life is nasty, brutish and short).
TV worlds like Sense8 fit - explaining their cult popularity: a sprawling, possibly senseless world, but it hints at depth and inclusion. Written about Becky Chambers in the past, and her #SpaceisforEveryone ideology would also fit. But I can't argue that either have hit Tolkien/Potter-size.
Curiously, the one property that does fit would be The Walking Dead, which, lest you smirk, is still, seven seasons in, the most viewed show in the world. There's no singular focus like GoT: Rick et al. are very clearly just a few of the many co-existing survival stories. And, like Harry Potter, there's space for everyone. Having your neighbours go zombie isn't quite the same thing as getting the owl from Hogwarts, but it is still an engaging, escapist, universal call to arms.
On the other hand, screw worldbuilding
Games designer Alexis Kennedy builds on the above point, explaining how we've misunderstood Tolkien:
You can find any number of well-intended world-building guides which say, menacingly, something like 'Start with a map and a timeline'.... [I]t isn't what Tolkien would have wanted. His attitude to worldbuilding - which he sometimes called 'sub-creation', or 'secondary creation' - was summarised in a piece called 'On Fairy Stories':
"The realm of fairy-story is wide and deep and high and filled with many things: all manner of beasts and birds are found there; shoreless seas and stars uncounted; beauty that is an enchantment, and an ever-present peril; both joy and sorrow as sharp as swords. In that realm a man may, perhaps, count himself fortunate to have wandered, but its very richness and strangeness tie the tongue of a traveller who would report them. And while he is there it is dangerous for him to ask too many questions, lest the gates should be shut and the keys be lost." [my emphasis]
Tolkien didn't build a language or make a timeline because it would create 'joy or sorrow as sharp as swords'. He built a language because he was a philology nerd who loved languages, and then he put it in his invented world, which we remember because of that world was full of memorable sights and thematically aligned stories and teasing hints about something greater. He made a timeline (and revised it I don't know how many times), but only because later on he wanted to make all the things he'd written to hang together....
[W]e all need all these things to hang together! Especially if we're building franchises that might end up with legions of fans and Internet continuity arguments and wikis. And one way to make these things hang together is dates, king lists, invented languages, and lots of maps. But these things aren't the core of secondary creation. They're plumbing.
Kennedy goes on to explain that building great worlds isn't about the timeline, it is about the evocative details: "scavengers playing acoustic guitar around a fire; it might be the way backstory clicks into place; it might be the emotional undertone of an invented proverb; it might be the tilt of a ringed planet against the night".
Maybe the success of Tolkien (and Potter) is because these world worlds are a framework - glorious details and subtle hints, but with plenty of space left for our own imagination?
Other links of things that are existing on the internet but mostly about pulp fiction
The strange and wonderful bibliography of pulp author Marijane Maker (Julia Carpenter)
Peter O'Donnell and his two imaginary women (r/fantasy, by yours truly)
The correspondence of James McKimmey and Philip K Dick (and Jason Starr) (LA Review of Books)
There's always time for L. Ron Hubbard's fiction and the rise of Scientology (Longreads)