Mystara and planes
by RipvanwormerThe World Serpent Inn is also described in a web enhancement for the Manual of the Planes.
One big difference between Planescape and Mystara is that in Planescape it's very important that entropy isn't an inherently evil concept - the Doomguard are an inner planar faction who protect the natural process of universal decay. Some of them interpret this in an evil concept, but not all. In Mystaran terms, there are as many Doomguard dedicated to the Sphere of Time (or even Matter) as Entropy.
Mystara's Limbo somewhat resembles the Fugue Plain that the souls of Forgotten Realms characters travel to after death. In 2nd edition AD&D, the Fugue Plain was a realm in Hades, and Limbo could fit in the same way.
As far as the main planes of each sphere go:
Arborea: Energy, Thought
Beastlands: Thought
Elysium: Thought, Time
Bytopia: Matter, Thought
Celestia: Matter, Thought, Energy
Arcadia: Matter
Mechanus: Matter, Time
Acheron: Matter, Entropy
Baator: Entropy
Gehenna: Entropy
Hades: Entropy
Carceri: Entropy
Abyss: Entropy
Pandemonium: Entropy, Thought
Limbo: Energy
Ysgard: Energy, Matter, Time
Arborea: Energy, Thought
Outlands: Time (Pandius rests at the base of the Spire, and this region belongs to no Sphere).
Individual realms might be ruled by any sphere, or no sphere.
And then the Inner Planes:
Positive Energy Plane: Energy, Matter, Time, Thought.
Radiance: Energy
Lightning: Energy, Thought
Steam: Energy, Time
Mineral: Energy, Matter
Fire: Energy
Magma: Energy, Matter
Earth: Matter
Ooze: Matter, Time
Water: Time
Ice: Time, Thought
Air: Thought
Smoke: Thought, Energy
Ash: Energy, Entropy
Dust: Matter, Entropy
Salt: Time, Entropy
Vacuum: Entropy, Thought
Negative Energy: Entropy, Time, Thought, Matter, Energy
Demiplanes:
Electromagnetism: Energy
Time: Time
Ravenloft: Entropy, Thought
Other Planes:
Region of Dreams (Curtain of Colours): Thought
Temporal Prime: Time