Suppose your fantasy multiverse has multiple material planes, each a layer of a hypersphere...

• Each layer is a 3D spatial slice of the 4D Glome that encompasses many parallel universes. Each universe has a finite thickness in the 4th dimension, therefore the number of parallel material planes is finite (perhaps dozens, hundreds, thousands; no one has been to them all).

• Each layer is connected to its two adjacent layers via secret places (caves, ley lines, and 'haunted places') and the occasional artificial passage created by magic.

• The layers near the equator of the hypersphere are the largest universes but are also the least magical; our universe is one of these. As one goes higher or lower in layers, the internal volume of each layer decreases but the power of magic also increases. None of these layers are more inclined toward good, evil, law, or chaos than any of the others.

• There are planes that exist outside of this hypersphere but they are the domains of the gods, their supernatural servants and messengers, and the departed dead.

So: logically there must be one "uppermost" layer and one "lowest" layer of the material hypersphere, both very small but positively inundated with magic. What would you put in these layers (and perhaps their nearest neighbors) as a world builder?