With the classic cosmology setup of the AD&D universe, the setup is Elemental - Ethereal - Prime - Astral - Divine...
I'm considering a cosmology where there is no Prime Material Plane. Instead, we have the Elemental Planes everyone knows of simply because they're the foundation for existence... They surround and protect the Plane of Time, which holds a greater power than any god should ever be allowed. The Deep Ethereal connect the Elemental Planes to the Border Ethereal on all the other planes, but in this cosmology it's rather hostile to life - streams of raw elemental substance whipping across the Deep Ethereal make it dangerous to try to navigate.
The rest of the Planes are generally Prime-like, but the gods of each plane live on that plane, possibly even amongst their followers. Some planes are dominated by a single race (picture the plane of Orcs, or the plane of Trolls!), while others are the scenes of strange events. Some are dead planes, without any life at all; others are magic-dead, and still others wild magic. It's incredibly easy to create artificial planes, as well - a mid-level mage or cleric could do so, or they could be hired to create one for someone else. Of course, they have to be stabilized constantly and maintained carefully - without the natural balance of a normal plane, they can easily go off-balance and destroy themselves. An experienced 'planeshifter' mage - one who specializes in the manipulation of planar pockets - can build remarkably stable pocket planes.
The Astral Plane links the non-elemental planes into a coherent whole, although very, very few know of the existence of it - partly because most folk are too interested in their own worlds to look outside them, and partly because the Astral tends to slowly erode the minds of those who travel it. Spend too long outside a normal plane in the Astral, and you'll quickly find your awareness and sanity being stripped away by the cloying vapors of the planes.
Demiplanes form all the time, born when some fluke of probability pushes a knot of Astral mists across a boundary to the Ethereal, where a rush of elemental energy infuses the knot and gives birth to a new plane. Travellers are advised not to enter such a newborn plane, as it often has conditions similar to the heart of the Plane of Fire - volatile, explosive, and deadly, as the elements war with each other to find a balance.
Other oddities include emotion-based planes of existence, planes where life once existed which are now reigned by the undead, and frightening planes where the very laws of magic and science break down or warp in strange ways.
Some planes are still stranger, such as a world where the force of gravity draws up into the sky, creating a world where everything hangs from the planet or flies through the skies...
Opinions? Ideas? Commentary?