The Cosmos

 Returning to the topic of my Default D&D world, we talk about the Cosmos of D&D.

I have mentioned previously how I would prefer to keep the nature of magic, the supernatural, and the fantastic less defined and codified.   However, from nearly the start, the mechanics of D&D imply a certain structure to the cosmos.  Cleric and Wizard magic work differently, spells make mention of other planes, and beings such as higher powers and elemental.  The published versions of the game itself have set out a specific nuanced cosmos.  Finally, the point of this exercise is to define a default D&D world that tries to adhere to the various versions of the rules as published.

First, I love The Great Wheel.  It is an intriguing design that reflects the rules and concepts of the game (such as alignment).  So, one may ask, if I like The Great Wheel, and I want a default world that honors the game rules, why am I even worrying about this?

As I said, I like to keep things mysterious. Also I want to leave room for other ideas; such as the underworld being a place one can physically journey to ala Greek Mythos

Related to the notion of keeping things mysterious, I tend not to use "the planes" as adventuring locales (at least not in the traditional sense), so this informs my thinking.

My basic concept comes down to this, a game world, its all true.  In short, we are tiny mortal minds to make sense of realms beyond not only their understanding, but even their own physical ability to experience.  Even the greatest of sages and wizards are but blind men attempting to describe an elephant.

To one observer, the planes are neatly organized in a great wheel and rings of the inner planes.  To others, the river Styx makes its way through the elemental chaos on its way to the abyss.  One may liken it to ones experience of a state if one traveled exclusively back roads, vs highways and byways.  The state is the same, it is your limited perception of it that is at issue.

In practical terms for game play this means there are a number of places/phenomena mortals of the prime material have common comprehension of.

Astral Plane: A transitive plane between "here" and "the afterlife", and for the applicable spells.

Ethereal Plane:  A plane slightly out of shift with the "real world", also a transitive plane to other material planes.

The above are less places you can visit, and more margins of existence you can utilize.  

Co-terminus Planes: Shadow planes, dream planes, faerie realms,etc.  These are other "nearby" planes that are similar to the "real world".  These are handy adventuring locals.

Planes where Elementals come from: Cuz they got to come from some place, these spill over to the "real world" in volcanoes, oceans, etc.  These planes are inhospitable to mortals expect in "overlapping" areas or specially created places like the City of Brass. The largest mercantile of the living realms.

The Afterlife:  The Great Wheel, the Heavens, the Abyss, this is a place beyond mortal comprehension, and any structure it appears to have is the efforts of Sages to make sense out of that which they can not fully experience.  This is why Valhalla and Tartarus, Asmodel and Vecna can exist and seemingly intersect... or not.

SIGIL:  This city of the outer planes, it exists because it was the only "safe"/neutral place to be in the realms of the gods.  It is the most "hospitable" place in "the afterlife". I have previously touched on the role the gods may play.

Spelljammer: love the setting and its solutions to keeping worlds separate, and some of that informs how a treat the planes. Almost choosing to keep a world and its whole cosmology in its own "sphere".  When I do think about bridging the gap, I think I would replace the Pholgiston of the setting with something more akin to interstellar space.  The Crystal spheres of worlds, sitting in the darkness of traditional space, almost like dyson sphere.  This would allow traditional/real world views of space to live along side D&D Fantasy space.  But the ultimate form it would take depends on the needs of the game I want to run, after all "all things are true".