The world of Glorantha, the setting for a few not-well-known RPGs and a slightly-better-known PC/tablet game, has a ridiculously detailed world, and it’s one of the few (Western) fantasy worlds where it doesn’t feel like huge chunks are Tolkein-descended.
It’s really interesting, because a lot of Tolkein’s Middle Earth was formulated around his love of linguistics, and the books are in some ways a means to play with those ideas (along with some introspection about his theological beliefs).
The setting of Glorantha, on the other hand, was written by an anthropologist trying to work out how cultures formed, and shifted and interacted, with a heavy focus on how those cultures tell stories and form myths and religions. The thing a lot of fantasy writers miss is, aside from a creation myth, there’s a lack of “just so” stories. Or the gods have too much agency, so it ends up not mattering who the “fertility god” is in the pantheon.
Glorantha is really, really good at writing its gods in that vague, fuzzy, metaphorical, self-contradictory way that most old myths are. It then has the audacity to assert that all of these different, contradictory things are true simultaneously in the confines of the world.
It also has, roughly, a culture for every different archetype of real-world religion. You have a lot of different animist and polytheist religions, some monotheists that are inspired from things to Zoroastrianism to Christianity. It’s really diverse and it does all of them well.
Even its original creatures are straight out of something weird an old culture trying to make sense of the world would come up with. There are creatures called Broos that rape everything, and whatever they rape becomes pregnant and produces more broos. Note that I said “everything”, this includes inanimate objects. But the key thing is that they also spread disease, so they serve purposes both as fantasy creatures, but also plague-metaphors, so if “broo are coming” it kind of metaphorically means people are getting sick (but also literally means goat demons are coming).
It avoids some of the stagnancy of Middle Earth, with the caveat that the setting is set up so that things are a bit tricky with regards to technology. The only metal better than bronze is iron and it’s extremely hard to get and both metaphorically and literally death. That said, there have been things as advanced as a sort of primitive robot in the setting, but it didn’t end well and in the current “age” in the setting things have backslid considerably because the last technologically advanced people tried to alter reality to their whim and it didn’t end well.