I am once more happy to have everything spelled out for this thread and recorded to be able to read it again. After these weeks of fully working out the finer details of AW and doing a lot of theoretical work on campaign structure, I had lost sight of the weird wonder that inspired me at the very beginning. As the structure became more clear, the details in my mind faded to something more generic. There is still the instinctive tendecy to make the world the most pretty and nice I can think of, but that goes directly against what make a setting conductive to interesting stuff happening.
Nice is the opposite of exciting.

Looking back at the first post, two things that I didn't really pick up on are the exotic grandness of Dark Sun and the mystical gravitas of Dark Souls. While the situation in settled areas is important, it's the threats that can come out of the wilderness that are supposed to be the stars of the show. And I have not really created much of that so far. Distinctive architectural styles for ruins and exotic farm animals is barely a start.

One thing I am pondering right now is to go back to an old idea and base the wilderness on some of my favorite places from the Planescape setting: The endless wilderness of the Beastlands and Arborea, the endles chaotic caves of Pandemonium, and the dark and stormy volcanoes of Gehenna. It's certainly a starting point.

What I think is really missing and needed is a solid systematic foundation for the spirits. Making them up at random case by case is not going to do it. Mystery and ambiguity are good and needed, but if people are meant to see themselves in the middle of the food chain, there needs to be at least an impression of a hierarchy of power extending above them. Not a divine beurocracy, but a general pecking order. People don't matter in the bigger picture, but there has to be something that does matter to the spirits. I think it's better if that something is beyond mortal understanding, but there needs to an impression of something being there.

I don't have a lot there yet:
I am ambivalent about having dragons. If I use them, then they would be like Chinese lung, not European dragons. Each one a lesser deity in its own right. Dragons don't talk. If they are to communicate with mortals at all, it would be purely telepathically. But maybe not even that.
I definitely want to have nymphs, including dryads, neirids, and oreads. They will also be pretty powerful and can be the local gods of smaller towns. Their true bodies are rivers, mountains, or ancient trees, but they have an ability to manifest humanoid bodies to interact with mortals. But such interactions should still be weird.
Naga are another must have. I think they should be mortal to make sense as a distant civilization, but very long lived and with magical powers.
I really like sidhe as spirits that look humanoid but are really quite alien in mind. I can see them as being common relative to the other much more rare and unseen spirits, but don't have a specific culture for them yet. They would be the ones to inhabit the non-ruined castles in the wilderness. But I don't want them to have full towns or cities and having to rely on farming. I'll see what I can come up with.
I think I want to have giants, trolls, and harpies in some way, but making them explicitly spirits and giving them some magical powers should really make them much more interesting. Particularly in a campaign where NPCs don't have individual stats. I like the idea of trolls seeing mortals as somewhat equal beinga to themselves, talking normally, visiting towns, and occasionally taking mercenary work. But they also would have to be odd in ways that don't make them simply explaining everything about the world of spirits to mortals.
With the Spirits of Beneath and Beyond, I think they should appear so rarely that each of them can be pretty much unique. And they are so completely alien that mortals aren't even able to tell if one of them is effectivly an animal or hyper intelligent. To them, all mortals are harmless critters that can be ignored unless they want to eat them or become a nuisance.
I think with working these out to more detail and adding some magical animals, it would already be a decent starting selection.