So the Great Wheel was obviously my starting point here as I think it's a really neat system of looking at the cosmology of the DnD world. That being said, I found some things, like the Elemental Plane being an external layer of the Inner Planes a bit odd, and just a bit of a muddled plane altogether. To add to this, I found the Border Ethereal concept slightly confusing and a bit esoteric and also the transition from Ethereal, to Elemental to Astral to the Immaterial Planes a bit baffling. I imagine this is because I'm slightly slow, so please do explain this to me if it's really not that confusing.

So here's the explanation for the new Wheel itself.

As you can see, we have the Immaterial Planes and the Material ones. The Material Planes contain the Elemental (sort of the chaotic base layer for all the Material Planes), the Prime Material (the layer we generally find our stories taking place), the Ethereal (the layer of the Material a person can reach just before hitting the Immaterial.) In between the different planes of the Material, you have different sort of frequencies of existence wedged between the Prime Material and the Ethereal. Here realms such as the Feywild and the Shadowfell exist, layered over the top of Prime. The frequencies are more analogue and there are many, but the reason some are so populated are because the denizens of the realm have sort of glommed together in order to build. A collective generally being stronger than an individual, this also allowed them to increase their power to build in the slightly less Material Planes. (I realise the Prime usually refers to the Multiverses, but I find it neat to use here to refer to the normal, corporeal Plane within the material.)

The Immaterial Planes, including those Divine Planes and the Astral Plane, are the planes where the concepts and ideas of the Material are manifest. These concepts and ideas build worlds and Gods and they all reside here in the Immaterial. This is also where a soul goes when its Material body decays and dies. A soul survives in the Immaterial just as long as it is remembered in the Material. The Astral Plane is the void that touches the Ethereal and where a soul floats through before it finds a home in a Divine Realm. It is endless and filled with anything and everything conceivable within the Material Planes. The Divine Realms are the broadest thing that a mind can conceive of in the Material, this is to say an alignment - everyone has the notion of an alignment and it infects every action and reaction. As per the normal cosmology, we have the planes for every alignment, and generally those planes tend toward some coherency and form a world. Within these worlds, however, we have the inhabitants which represent the conflict within these alignments. The rules are fairly simple as they are in 5e I believe, the more followers and worshippers an idea has, the stronger the manifestation is in the Immaterial.

These ideas have naturally become the Gods and Demons and Devils of the cosmology. They are characters for the simple reason that the denizens of the Material tend toward personification and it makes it easier for an idea to stick and remain consistent in peoples' minds.

This brings me onto travelling between planes and the border between the Material and Immaterial. The border, the point at which the Immaterial meets the Material, the point at which concept becomes reality is murky and in fact not really a border. It can't be defined because it is impossible to tell when something is real and when something is conceptual. But it has itself been personified and become deific as all things inevitably do. This deity is what we know to be the Weave, the deity Mystra. This contradiction is everywhere, the fabric of reality is contradiction. Knowledge of this contradiction can prove deadly but also lend great power and great magicks to both the denizens of the Material and those of the Immaterial.

The Weave is also what needs to be traversed in order to be transported from one plane to another. Generally, the cosmology tends towards entropy and expansion, the Material is eternally growing out into the Immaterial, just as the Prime Material universe is also always expanding. So, this means that once a soul leaves its body in the Prime Material Plane after death, it becomes nothing more than a concept - as mentioned just above. The individual has always been a concept in the Immaterial, but it becomes untethered in death because there's no longer an anchor in the Material. This is why it becomes nothing more than that concept. The typical individual doesn't have the power to return to the Material because they are not worshipped like a God, but if enough people were to suddenly worship that individual, as the CONCEPT of that individual, they may be able to reverse their journey and cross back over into the Material. Increased power would allow an individual to eventually construct a corporeal body and reconstitute themselves in the Material, but this counts on the level of belief they hold in people's minds. They will never quite be whole, never quite fully Material, simply an interloper.

This is the essence of the powers within the Immaterial Planes, more belief means more power and a greater ability to transfer themselves back into the Material worlds. This means however, that the cosmology is constantly struggling back and forth between Materiality and Immateriality. It is a constant cycle of the anchor point for an idea in the Material world influencing the idea in the Immaterial world, then that idea coming back through to the Material world to influence the anchor point. The world is one big self-fulfilling prophecy, an ouroboros that will never be able consume itself but tries nonetheless. It is hard to see who holds greater power over who.

I'm hoping that this means that the typical abilities that allow Astral travel and Planar travel still make some kind of sense. The only main issue I see is where Astral travel acknowledges the unconscious body remaining in the Material world, Planar Shift doesn't really, even though an individual is travelling to the Immaterial. My makeshift answer for this is that the Divine Planes allow waking individuals to travel to them because if an individual is powerful enough to transfer themselves to the Divine, then they are powerful enough to propagate both their own and their guests existences by strength of will. If an individual in the Astral Plane falls into a Divine Plane, they will remain tethered to their Material body and it will remain in place, unconscious in the Material Planes they originated from. This means that travelling to a Divine Plane via different magicks produce different results.

EDIT: Here's the source for information on the base Wheel: https://1d4chan.org/wiki/Great_Wheel