Prime is the Elemental Plane of Void - a What If...
by LoZompatoreVery interesting approach!
It reminds me of PC1 mythology about the ancient Immortals from which the wee folks descended. At that time Enropy was dispersed into four chaotic Spheres (one per element) and a single Sphere of Integration that, ultimately, "won" the conflict with the four elemental chaotic Spheres and led to the current order among the Immortals.
Quoting from PC1 (DM's booklet page 23):
"Once upon a time there was Chaos. Not a few minor disturbances here and there, like one suffers from a small tribe of wood imps; no, Chaos filled the Multiverse, and was as natural as order is today. The Immortals then were different; four Spheres of Matter, Energy, Time and Thought comprised the Entropy that spanned the multiverse. They all opposed the Sphere of Integration, or Life as it may be called, because they feared the new order it would bring.
You see, life arose from those four chaotic elements - it was their legitimate offspring. But Chaos hated and feared its child, because Life would ultimately bring the order that would ultimately subjugate Chaos and conquer the Multiverse.
That did come to pass; we see that result today, and the story is now reversed. Entropy is now but one Sphere, and order dominates the other four; but from order naturally stems stagnation, as from life, death. This is why the Spheres of our age fight Entropy (or Death, as it is also known), and fear it. It is their child, even as they were born of it; and as surely as they defeated it as their parent, they too will succumb to it as their offspring and nemesis.
This is the Great Circle of the Multiverse."
The arrangment of the Prime and Elemental Planes may mirror such changes among the Spheres, in particular with respect to their content and boundaries.
In the Immortal Set it is said that the Elemental Planes are finite in size, well explored and leaving few surprises to the casual traveller. This may reflect the order imposed on such planes by the four "tamed" elemental Spheres.
On the opposite, the Prime is boundless (thus impossible to fully explore) and a mix of elements which also include a pretty large amount of Entropy in it. Such a share is currently in equilibrium with equal shares from the other Spheres but, from quote above, it seems that one day Entropy will ultimately be predominant in the Prime.
Moreover, the absence of boundaries and the infinite size of the Prime, seem to me necessary features to ensure that the (finite) four elemental planes will never be able to fill it with elemental matter and impose an order in all the corners of the Prime.
In short, I guess that the current equlibrium of the Spheres in the Prime is a fragile thing, definitely unstable and not destined to last over time.
Maybe in the ancient era of Chaos the four elemental planes were infinite in size, and the Prime was a finite, void plane in which the Sphere of Integration was able to establish some kind of order and control. From this base of power the Immortals of Integration were able to defeat the elemental Immortal of Chaos. In doing so the Immortals of Life were forced to accept Entropy in their original Plane, leading to the corruption/destruction of planar boundaries of the current Prime (by the way, it could be interesting if, somewhere very far away in space, there are the relics of the orginal spatial boundaries of the Prime regions of corrupted space like the broken shell of an egg, where Entropy is predominant and strange phenomena occur. Outside such regions, of course, there are other stars and planets and galaxies and so on).
Finally there is also the enigma of the draedens which, as per Immortal Set, despise the four elements and like roaming the endless void of the infinite planes. Are they the original denizens of the void Plane which is now the Prime?