Gary based the cosmology for Mythus in large part on his old DnD cosmology. Which in large part he got from a proposal from Stephen R. Marsh. Now Gary did present his cosmology in a 2 dimensional form, but it has occured to me that in that existence is 3 dimensional, then the cosmology would be 3d as well. In addition, that the various planes would not necessarily be separated as represented in the illustration. So I'm now going to give you may version of affairs.
In my scheme of things the reality we know of as the worlds of Dangerous Journeys covering modules such as Mythus and Changeling, plus whatever else is out there, is three dimensional. Or it can be described as having two axis, the vertical and the horizontal. The core is a sphere, with the outer planes constituting a shell.
At the heart of this cosmology is what we call the Prime Material. In all honesty, the Physical is but part of it, but it is the part most readily apparent to us. In total there are the Mental (Astral), Physical (Material), and Spiritual (Ætherial) planes. Each corresponds to an aspect of life. In addition there are the planes of Ærth, Air, Fire, and Water, but in worlds such as Changeling and Abyss these are understood to be more in the nature of natures than in actual things. To a Changeling Scholar or Abyss Scientist the terms used are Solid, Gaseous, Plasma, and Liquid respectively, and even on Ærth that is how they are seen as well.
The Outer Shell
Here is where it gets a bit more complicated. The outer planes are more abstract, more conceptual than is the core. By and large those below tend to be unpleasant places indeed, while those above seem rather nice. But keep in mind that in as much as good and evil are not seen as things, you can still find higher powers doing ill, while lower powers can be charitable.
The Positive and the Negative are found here, as are the Celestial and the Abyssal. Indeed, while connected to the Mental or the Spiritual the Astral and Ætherial planes extend here as well. (It's also said that the Physical has it's presence here, but most consider that wild speculation.)
Still doors and gates between the planes are known, and used more often then one might think. The Blessed and the Accursed are known to hold conferences on empty alternatives to Earth, with powers and potencies going slumming in cities such as Ys and New York. Though even the Accursed have banned visiting certain Ys locations, just too dangerous.
It must be noted that the worlds of Dangerous Journeys are not our worlds no matter what Mr. Grumbolt may have told you.For the DJ reality has one property our reality can't. Namely that Magick can exist there, for the nature of spacetime is such as to allow the disturbance in spacetime that can be manipulated by those with the talent. But that's a subject for later.
If you feel like doing a 3d model of the DJ Cosmology and adding to what I wrote here, go right ahead.