Going with Havard's idea of the plane of Shadow being an Outer Plane (it's an inner Plane in the Great Wheel Cosmology), I had this thought of creating a unique version of the plane for Mystara with a different name. I was thinking either the Veil of Shadoes (in honor of Stranger Things) or Umbral (inspired by the Umbral Creature template for 3E).
Umbral/The Veil of Shadows (Multiverse Dimension)
by Robert BlezardAddress: Mirage-Umbral ?
Size: Cluster
Appearance: TBD
Dominant Sphere: ?
Physical & Magical Laws: Is considered magically morphic when it comes to shadow-based magic. Gravity is normal for any "body" large enough.
Passage of Time: Same as the Prime Plane.
Spells/Powers Needed to Survive: None.
Spells/Powers Needed for Movement: Flight powers or teleport spells are required to move in the atmospheres of the plane's worlds (whatever we decide they will be) and between worlds (through an airless void). On a planetary surface, normal movement works.
Effects on Mortal Magic: Shadow magic is enhanced. Such spells are twice as powerful as they would be normally. However, any spells used to generate light or fire may fizzle when cast on Umbral. (It is a Spellcraft check for 3E. I'll leave it to others with better knowledge of BECMI to come up with a D&D ruling.)
Effects on Immortal Magic: Will depend on the Dominant Sphere chosen, if any - KF
Effects on Immortal Special Powers: None.
Native Races and Monsters: Undead of all sorts, including the undead shadow. Shadow-touched animals and plants (dark and twisted). Nightshade, Shadow Mastiff (maybe), Ecalypse and Umbral Banyan (see 3E Manual of the Planes), and another creature called the Dusk Beast that is often found near vortices. There is also the Shadar-Kai, as an option, from the 3E Fiend Folio. They would live in dark, twisted forests and the underground realms of Umbral's worlds.
What should Umbral look like considering the Plane of Shadow is described as being black and white (no color at all). It is described in the 3E MotP as being the toxic plane of darkness and power, as well as the hidden place that hates the light. It is also described as the frontier of worlds unknown, which points to the Plane of Shadow's use as a transitive plane. For Mystara, as an Outer Plane, it wouldn't be truly transitive, although gates in Umbral might lead to darker places such as the Pyts and the Nightmare Dimension.
For 3E, the Plane of Shadow is listed as Mildly Neutral-Aligned, with regions known as the Darklands having minor Evil Traits. Perhaps some world are touched by Entropy.