From the void came thought, from thought came motion, from motion all that is seen and unseen came to be.Anaxiletus - Motus, Primus, et Deos
It is believed that the Astral Sea and the Ethereal Mists have always existed. The Gods do not speak of it, the Titans question the importance of knowing their origins. However, all seem to agree that where the Astral Sea borders the Ethereal Mists, a river began to flow of an admixture of the two. This Affluviam is considered the first cause that generated the universe.
As the Affluviam flowed into the Ethereal Mists, the seven elements of the Central Wheel were formed. The first element, Bosk (Wood) was formed and as the flow continues, the rest came in order: Air, Fire, Earth, Stone, Metal, and Water. Where the elements bordered each other, came Serrago (Wood and Air), Smoke (Air and Fire), Magma (Fire and Earth), Gemstone (Stone and Metal), and Rust (Metal and Water).
Where Wood and Water met, however, a branch of the Affluviam changed course and Stroth (The Great Marsh) came to be. From the Great Marsh, life began as the start of the Inner Wheel. From Birth (the Forge of Souls), the Affluviam coursed into Growth, then to Decay and then to Death, and emerging back to the Central Wheel through Threst (Compression).
Threst is the ultimate destination of the physical body where it is transformed into a stone-like material before cast adrift in the Astral Sea. The dead gods and titans floating in the Astral Sea are proof enough that the Threst exists.
The mind, however, joins the Affluviam to journey through the Ethereal Mists and into the Empyrean Wheel, the lands of the deities (Gods, Titans, and Powers). Some minds never escape the Ethereal Mists choosing to form their own ethereal home. The mind take ethereal form as ghosts and other spirits. They in turn form the myriad Ghostrealms that stretch the infinite mists.
Beyond the Empyrean Ring lies Ad Lucem (Towards the Light) and Ad Umbram (Toward the Shadow). Light and Shadow admix through all of creation drawing sentient life toward one or the other. Some are drawn to a balance. This is where the ideas of Good and Evil, Law and Chaos come from. Their purest forms do not exist, even in the Empyrean Ring.
The known world exists in the center of Central Wheel where the elements, life, light, shadow, the Ethereal Mists, and the Astral Sea converge. Other worlds with different elements and different life flow with the Affluviam throughout the Astral Sea.
Beyond the Empyrean Ring lies only thought. The alien Ngngilligan Horrors and the Ninian Angels are believed to emerge from there. Illic furor est.
This is the foundation of the how and why of this universe. Instead of Gods, there are deities that comprise the Gods, the Titans, and the Powers. The idea of the Light and Shadow are meant to imply that Alignment as a game concept doesn't exist. The concepts of good and evil, law and chaos exist, but not to the extent that each concept has a common language or that magic can divine these concepts to produce an effect. As such, Clerics still exist and worship of one of the deities provides access to various domain spells per the standard rules. The Powers, however, can also serve as patrons for Warlocks.
The Njnjilligan Horrors are a stand-in for Cthulhu. Lovecraft is problematic and I don't want any part of it. The idea of non-Euclidean horrors, however, can serve as useful game material. I imagine cursed items, Phrenic Scourges, various plagues, and more could have roots here. The idea of something that is "beyond the gods" can also serve as a source for challenges to very high level characters.
The seven elements at a base level provide different kinds of elementals. It adds three more dialects to the Primordial language. The goal here, though, is to lay the foundation for the elemental bloodlines for characters. The elemental planes are home to creatures of all kinds, including sentient species like the Haizawer (Air), as well as animals like the Tineshuna (literally a fire-fox), as well as kings, lords, and beings of great magical powers. I do not believe that Genasi are OGL, but I don't have djinn in my world. Ancestors of those with elemental bloodlines can be a variety of sentient beings.
The Marsh as the beginning of life still provides a plane of Life, but nothing so overpowering to cause radiance damage by just visiting. It is earth-like, but serves as a gateway to any place on the Central and Inner Wheel. The Threst is my attempt to explain why there are dead gods floating around the Astral Sea. Basically, everything that died is transformed into the stone-like material of the dead deities floating in the Astral Sea. Anything Medium sized or smaller isn't encountered: the compressed body doesn't last very long, becoming dust. Elemental Compression is weird, true. I'm still working through what adventures there would be like.
My favorite thing so far, though, is the Ethereal Mists. There can be worlds there where the living and the dead can co-exist. The Ethereal Mists are still the fundamental source of all matter, so the thoughts of the dead provide a ghost body. Those of strong enough will could gain physical form as well. I imagine these worlds can be terrifying, but I also wanted to provide places where characters can visit the dead and talk to them.
One last thing. Anaxiletus is my NPC that is the great historian and philosopher of this universe. I haven't explored more about him yet. Anyway, those are my thoughts so far. Let me know what you think.