An attempt at a comprehensive multiversal structure
by David KnottOkay, here is how I would design a multiverse in which Mystara and the standard AD&D cosmology could co-exist:
The core of the multiverse is the Ethereal plane, within which exist many meta-planes, planes, and demi-planes. At its centre are the Inner Planes, which consist of the four Elemental planes, the Positive and Negative Material planes, and various quasi- and para-elemental planes. As the source of physical reality, these Inner Planes are the only elements shared throughout the multiverse.
The difference between a plane and a demi-plane within the Ethereal is primarily a matter of degree in terms of size, age, and stability. Generally, a demi-plane is small, young, and subject to complete destruction by outside forces, while a plane is larger, older, and so well established that its destruction is inconceivable. What they have in common is being cut off from anything that might be called an "Astral Plane" -- the only Powers that can affect them are those native to the Ethereal and Inner Planes. Athas (Dark Sun) and Laterre certainly form such planes, and it is possible that Cerilia (Birthright) does as well.
A meta-plane, on the other hand, is connected with both the Ethereal Plane and something outside it. The two known meta-planes are the Light Prime and the Dark Prime.
The Light Prime is a single Prime Material Plane filled with phlogiston and crystal spheres and connected with the Astral and Outer Planes as described in the Planescape setting. No plane or meta-plane outside of the Light Prime, however, has any direct connection to these Outer Planes. Greyspace and Torilspace (the sphere of the Forgotten Realms) are certainly within it, as was Krynnspace (the sphere of Dragonlance) until mysterious events removed it to its own plane. Anyone with material from both the Spelljammer and Planescape settings has a thorough knowledge of the meta-plane known as the Light Prime.
The Dark Prime is centred on the mysterious dark powers of the Ravenloft setting. Within it are the planes of Darkness, Ravenloft, Gothic Earth, and Mystara, in increasing distance from the dark powers themselves. One feature that all of these planes have in common is that magic associated with aligned "Good" or "Evil" is totally ineffective anywhere in this meta-plane. However, while evil alignment is undetectable, evil actions still have consequences, whether in the form of "Powers checks" in Ravenloft and Gothic Earth or in the form of association with the Sphere of Entropy in Mystara.
While the Dark Prime is cut off from the Outer Planes accessible from the Light Prime, it has the most direct access to the Astral Dimension and the dimensions beyond it. This "Astral Dimension" corresponds exactly to what is called the "Astral Plane" in WotI, but it is clear that this Astral plane/dimension is a totally different reality from that described in the Planescape setting. Within this Astral Dimension are most of the home planes of the Mystaran Immortals, including the five major planes that are associated with each of the five spheres of Immortal power. The elemental "circle of dominance" in the D&D Rules Cyclopaedia describes not the relationship among the elemental planes per se but the interactions among the corresponding major planes in the Astral Dimension.
From the Astral Dimension one may access the Nightmare Dimension (home of the diaboli and other such beings) and the Vortex Dimension (blocked off by the Immortals). Anyone attempting to access the Normal Dimension from the Astral Dimension would first be drawn to the Darkness Plane in the Dark Prime. Nobody who ever entered that plane has ever left alive. Fortunately, most interdimensional travellers are able to detect the malevolent influences of the dark powers and so avoid that plane as well as the planes of Ravenloft and Gothic Earth. As a result, the Mystara plane is the obvious point of contact between the Normal Dimension and any other dimensions.
Finally, here is how I would resolve the "canonical" points of contact between the "Light" and "Dark" Primes:
1) The acquisition of Red Steel by some natives of Sigil is the easiest to explain. While there is no DIRECT access between Sigil and Mystara, INDIRECT access via the Deep Ethereal is certainly possible.
2) The placement of various Mystaran Immortals in the Outer Planes of the "Light Prime" has no basis in reality but is simply the result of an attempt by a native of Sigil at rationalising information that he received from a Mystaran.
3) The co-existence of the Norse Powers in both the Light Prime and the
Dark Prime has sages in both meta-planes stumped. The most likely
explanation is that they originated on Gothic Earth but sent forth avatars
into the "Light Prime" to assure their continued existence past the prophesied
Ragnarok that would certainly destroy them at some unspecified future date
in the "Dark Prime". In the plane of Mystara they incarnated as mortal heroes
who succeeded in attaining Immortality and then took up all the powers that
were associated with their true names. Whether they have succeeded in
establishing a power base that will survive Ragnarok remains to be seen.