(Concept) Equestria: Living World, Dead Worlds

The Equestria we see everyday is simply one face of a much larger universe, consisting of perhaps millions of worlds.

Among the worlds, one is very special, called the Living World, inhabited by living creatures. All other worlds are Dead Worlds.

By size, the Living World is vastly larger than the largest Dead World. The God of the Living World is Celestia. The pony princess Celestia is simply one manifestation of the true Celestia, albeit the main manifestation.

What the Dead Worlds lack in size, they make up in variety. The Dead Worlds are very different, obeying different physics, geometry, and have different visual styles. The God of the Dead Worlds is Luna. The pony princess Luna is simply one manifestation of the true Luna, albeit the main manifestation.

Spirits are crafted by Celestia from the energy of the Sun, then put into living bodies into the Living World. When a living body dies, the spirit would be catched by Luna. Based on what kind of life the spirit has experienced in the Living World, Luna would guide the spirit into a suitable Dead World, or create a new Dead World just for the spirit. Creation of new Dead World usually happens when a group of close friends die at the same time, or an extreme loner dies.

The largest Dead World, called Talassa ("the ocean" in Old Unicornish), accommodates almost a hundred million spirits. The smallest accommodates a single spirit. Spirits may request to travel between Dead Worlds, but since spirit transfers are costly, Luna would approve of a travel only once in a while. Instead, Luna encourages most communications between Dead Worlds to be done by letters and voice recordings only.

Very reluctant spirits might instead wander the Living World, or request a reincarnation. A spirit who has been reincarnated many times would end up with a very mixed memory of the Living World when they finally enters their Dead World. Very experienceless spirits (such as due to an abortion, miscarriage, etc) always gets reincarnated.

--------As An Example: A Story Under This Setting--------

This setting is applied to the main plotline of the Artline and Cosmia story series. In this main plotline, Artline and Cosmia became friends in the Living World.

The Living World in the main plotline is codenamed "Noir Ponyville" or "Noir Equestria" (in contrast to "Sunny Equestria" or "Season 1 Equestria"). In Noir Equestria, the world is more modern-looking, with gray concrete buildings, steampunk technology, and higher population. Ponyville is a small city with such things as a supermarket, a library, and a university named Aurora University.

Artline in Noir Equestria is an independent artist. Her parents are divorced and left the old apartment to Artline. She has no friend except Cosmia. Artline works in a bakery, and sometimes as a foalsitter, since she couldn't earn much with art. At the beginning of the story, she is around 18; she dies at around 24, right after Cosmia dies.

Cosmia in Noir Equestria is an astronomer and mathematician. Her father died and she is distant from both of her parents. She lives alone in an apartment near Artline's.
Cosmia and Artline met each other in a library, then developed a friendship over many conversations. At the beginning of the story, she is about 27; she dies at around 33, right before Artline dies.
The reason Cosmia is older than Artline in this story, is because Artline becomes independent right after high school, but Cosmia has to finish a PhD in Astronomy (not to mention a Master Degree in Mathematics).

The death of Artline and Cosmia comes after 6 years of their friendship. Cosmia receives a commission from the Royal Observatory to observe the Transit of Venus in Norway, so Cosmia travels there, with Artline beside her as a companion and assistant. There, Artline killed Cosmia before killing herself.

Luna sends them to a purgatory in the form of a frozen forest, so that they could repair their relation. Cosmia eventually forgives Artline, and they go to their final resting place, their Dead World.
Their Dead World is an infinite snow field, with a castle, and many dark forests. There are millions of books in the basement for Cosmia and Artline to read, and endless stationery for them to do art and math.

--------A Possible Variant--------

So far, in one story, this setting is modified, so that all spirits reincarnate until they are considered sufficiently experienced.
During life in the Living World, a spirit can only access memories accumulated in that particular incarnation. In Limbo and in Dead World, memories from the lives in the Living World are all accessible, but more distant ones are less detailed, murky like dreams.

--------As An Example: A Story Under This Variant Setting--------

This variant setting is used in one story, which deviates from the main plotline.
In this story, Cosmia died and was reincarnated as a filly called Miasma Nocturne. Several years after the death of Cosmia, Artline has found success as an artist, but still foalsits from time to time because she likes foals. One day, Artline goes to foalsit a filly, and it turns out to be Miasma. Artline is impressed by the similarity between Miasma and Cosmia, and becomes a regular foalsitter of Miasma. They become friends and meet regularly even after Miasma doesn't need a foalsitter anymore.
One Hearth Warming Eve, Miasma does not come. Artline finds out that Miasma has died of heart attack (Miasma has congenital heart disease). A few days later Artline also died, and the reincarnation continues.