I'm making my own cosmology for a game I'm running, basically there is the physical world & the "Ethereal Sands" a blurry, in substantial eternal desert of off-white sand. In the center of the sands is a enormous spire, a thin needle shaped tower impossibly high, though no one has been able to walk to it, it is always on the horizon & never seems to get closer.
This is the place when everyone goes to when we dream, that's why you wake up with sand in your eyes, & it is the place where dreams & nightmares come from.
The Sands are populated by massive wurm-beasts that "swim" through the Sands & attack anyone nearby. Also their is an entity known as the "Sandman" who is the lord & ruler of the place.
When in the Sands your Mental Stats fill in for your physical ones.
Str = Cha
Dex = Int
Con = Wis
Magic Users are very powerful here as this is the realm where magic originates.
If one can find a way to take a handful of sand from the plane, it can be used as a substitute for any material component as well as boost it's power.
Can anyone recommend me some books or rules to look at to flesh out the mechanics on this place?
If you call the lord of dreams the Sandman, you really need to read the comics of the same name, if you haven't yet.
For the whole mental=physical thing, you might check out the 2nd edition astral plane, which did this as well. And dreams, too.
An interesting mechanic might be stumbling into the dreams of other people, by accident. Entire elaborate cities and worlds, built from mirages and sand, where dreamers live out whatever phantasies they have.
While it is a little bit inspired by the Fade in Dragon's Age, it isn't very much like it physically (One large expanse as opposed to fragmented bits floating around) I also found inspiration from Dragonlance & Raistlin's journey to fight the Queen.
How much of it is a desert and how much of it is a magic-spirit world?
Do you what rules on dehydration? Or sands that reflect magic? How much of it will be like the Astral plane? How much like the etheral? Can the Sand-Wyrms go back and forth between the mundane world? Are they intelligent?
Which aspect is the most important?
The magic or the desert?
Personally it has a "power by ordeal" feel I enjoy with a dash of Dune.