The Multiverse and Mystara
by Jerry HovenanianSpelljammer
In Mystara Space there is different gravity, less air, and lots of cold. The way to convert the two of them is not to convert them. If you have read any of the Spelljammer stuff it is accepted that there are some pretty odd things in Wildspace.....why not a sphere that has different laws of fantasy physics? This would explain why there are no known records of Spelljammer incursions into Mystara. They would enter the sphere, the lower decks would get tossed about, they would lose all their air, and turn into spelljammer-cycles :) Ships leaving Mystara would only have to deal with the gravity issue and the fact that they travel so slow compared to spelljamming ships.
Planescape
The Prime Plane - it contains Mystara, The Dimension of Myth, and Many other prime worlds.
There are 3 roads (Rule of 3s?) leading from Mystara and the prime to other places.
1 The Ethereal Plane - it contains The Demiplane of Nightmares and leads to the Elemental Planes.
2. The Vortex Dimension - as it is fatal to mortals and immortals alike, could this be the home of The Old Ones? It leads to the dimension that the Beagle came from. This explains why the Federation never went looking for the Beagle. And if this is the case, are the Old Ones responsible for the Beagle going to Mystara? A test of the Immortals perhaps?
3. The Astral Plane - it leads to The Outlands and the Outer Planes. It also leads to the Mystaran Outer Planes and Old Alphatia. In some of the Planescape material they refer to the fact that there might be other hidden planes. You could easily say that Brynn, Entrem, Mirage, Draesten, and Pyts are some of these hidden planes. Old Alphatia could also be one as it is a very large thing that couldn't fit into a normal crystal sphere. It's extra-planarness would explain why everyone there has the abilities of a first level mage.
A Mystaran View
The Inner (Elemental) Planes | The Ethereal Plane (The Demiplane of Nightmares) | The Prime Plane---The Vortex Dimension----The Dimension of the Beagle (Mystara) (The Dimension of Myth) (Many other prime worlds) | ------Astral Plane | | | ------------------------------------------ Old Alphatia | | | | | | | Brynn---Entrem---Mirage---Draesten---Pyts | | | | | | | ------------------------------------- | | ---------The Outlands and the other Outer Planes