If you're playing Planescape, you care really about the 16 Outer Planes and the Outlands, less about the Astral Sea.
Indeed, the Outer Planes and the Outlands link to each other via portals. So characters can hop back and forth among all seventeen without ever entering the Astral Sea.
WotC have been very clear in 5e that the Planar Wheel is JUST A MODEL by mortal sages trying to make sense of the planes.
The map of the cosmology in the Players Handbook is the default model for 5e.
Even tho it is a model, WotC still needs to clarify practical questions.
Can player characters in a spelljammer ship dock at an island to embark into an Upper Plane such as Arborea, or into the Outlands to view and somehow get to Sigil?
A Higher Plane of Existence is not physically part of the same space as our world. Trying to say that Celestia is physically INSIDE the Astral Sea as opposed to connected to it is like trying to force a tesseract-shaped peg through a torus shaped hole but only being able to see in 2 dimensions.
In YOUR game you can make the Astral Sea or Astral Plane a physical space surrounding both the mortal plane(t) and other plane(t)s but that's not the defined 5e cosmology assumed by the core books.
According to the 5e Players Handbook, the default model includes the Outer Planes inside the Astral Plane. They are even specifically inside the Astral Sea.
The wildspace would be the part of the Astral Plane that overlays the Material Plane.
By inference, the Astral Plane overlays everything that exists in the multiverse, from the outermost Astral edge to the innermost Material center. The Astral Plane is everywhere.
In MY game, for example, the Astral Sea is a physical space related to Wildspace and Star Wars Hyperspace and IRL Outer Space; a space so big that distances and timelines become impossibly large and our tiny lives and world becomes essentially a blip in the fabric of existence,
My view is similar.
The Material Plane includes distant stars that are impossibly far away, like reallife.
In D&D, those distant stars have their own worlds, including planets like Athas, Eberron, Oerth, and Toril. Each planet is a separate campaign setting: Dark Sun, Eberron, Greyhawk, and Forgotten Realms respectively. It is unfeasible to try reach these stars by mundane means. The typical way to reach them is to enter the wildspace of ones own star system, then sail from there across the Astral Sea at the speed of thought to the wildspace of that distant star system. Those stars are all visible within the Astral Sea via their respective wildspaces. One dematerializes from one star system and rematerializes in an other star system, effectively teleporting.
Creatures in a wildspace can exist both the Astral Plane and the Material Plane simultaneously. These creatures age because of the planar properties of the Material Plane.
(There are no wildspaces in the vacuum of remote outer space, perhaps because there is little or nothing to overlap. Maybe it has something to do with gravity. In any case, altho the Astral Plane overlaps the empty space of the Material Plane, the Material Plane doesnt exert influence on the Astral Plane there, hence no wildspace.)
while the Elemental Chaos is like the Quantum Realm from Marvel - a hidden "inner space" so tiny that time becomes meaningless and the building blocks of creation drift between states of matter and energy.
You have the Elemental Chaos exist at the "quantum" level of atoms. That makes sense. Because it a "place" where Force, elemental energy (Fire, Cold, Lighting, Thunder), and unstable matter (Acid) are all in flux.
When I look at the 5e map, I see the main features as:
• Positive Energy and Negative Emptiness preexist the multiverse.
• Where Positive and Negative interact the Astral Plane of thoughts results.
• The Astral Plane causes and overlaps the Ethereal Plane.
• The Ethereal Plane causes and overlaps the Material Plane.
• The Astral Plane overlaps both the Ethereal and the Material.
• The Elemental Chaos is where Ethereal force becomes matter.
• This Elemental matter is the building blocks for the Material Plane.
Note, one can reach the Four Elemental Planes from the Ethereal Plane, because its Ethereal force is part of the Elemental Chaos and each Element is actually made out of force. The Elements are states of matter: earthy solid, watery liquid, airy gas, and fiery plasma. All matter derives from the fundamental forces. In other words, the Four Elements are made out the Fifth Element, ether, which is Ethereal force. Ether as an element is also called Quintessence.
The Elemental Chaos is like the Big Bang. It is a creation event. But it is ongoing, in a continual flux, where resulting matter can revert back into force and energy. That is true at the subatomic quantum level, is helpful.
Meanwhile, the Outlands in my game are literally beyond the raw edges of the universe, the space between all things and at the outer limits of our understanding
In the cosmology map, there is a ring from which spokes radiate to each Outer Plane. Perhaps this ring itself, or perhaps what is within it, is somehow the Outlands.
-- and where the Ruby Gate and its Shardmind guardians hold back the incursions of the Far Realm.
I kinda want to put the Far Realm entering thru a lower level of Hades − with the Gray Waste including Aberration imagery.
Meanwhile, "Hades" and everything belonging to any "underworld" of the dead become moreorless the same thing as Shadowfell in the context of 5e.
I see these as fundamentally related ideas to the 5e cosmology, but clearly the way I'm interpreting them and retrofitting 4e concepts and MtG concepts and building in my own ideas makes it different.
Because there are omissions in the official descriptions of how these planes relate to each other, DMs are somewhat forced to fill in the gaps, and reconcile incongruencies.
Planescape uses the Great Wheel Cosmology as a default, but remember that it's ONLY A MODEL. WotC don't have a financial incentive to define it further than that, especially given that the Forgotten Realms are their home-base setting. FR has a history of understanding cosmology in multiple ways, of plane lists changing, and that history wasn't wiped when the Spellplague ended and Abeir and Toril passed out of alignment with each other again at the turning of 5e a decade ago. The Forgotten Realms are a living setting, even if the exact details of scale have changed from edition to edition (seriously, the 3e and 4e maps of the Realms are SO MUCH SMALLER than 5e's version). And that means that the nature of Cosmology needs some flexibility to allow past interpretations to remain as a viable option for the sages to discuss.
It is the great point that Forgotten Realms, namely the default setting for 5e core, has a tradition of changing cosmologies. Talk about "Realms-shaking events"!
So they're probably not going to define the space between the planes further with Planescape; instead, they'd focus on the factions of Planescape and the campaign adventures of Planescape and the options of Planescape characters and what it means to play a game without a Material Plane as the central starting zone and place to protect from planar incursion. I'd expect close details on Sigil and character options, a starter adventure in Sigil, a larger campaign taking inspiration from PS:Torment, and a Manual of the Planes gazetteer that explores the creatures and factions and some key locations in each of the Outer Planes. I actually expect them to ignore the Inner Planes, the Parallel Planes, and the Transitive Planes when it comes to details, but to speak to them very briefly as also there.
Probably true, mostly.
But there are still practical questions that require clarification.
Can players in the Planescape setting, be in an Outer Plane like Arborea and sail out from a dock there into the Astral Sea?
In the Outlands, what is beyond the edge of the stable hub of the Spire? Is it the Astral Sea, the Elemental Chaos, more Outlands, or something else?
How does one get to a relevant dominion like Hestavar from the Outlands, or from Elysium? Does one get there by a spelljammer ship? Can one get there by a spelljammer?
I find it difficult to imagine a Planescape setting that avoids clarifying these likely adventure scenarios.
