Great Old One
Sanderson's Cosmology is pretty easy to understand. There's the Physical Realm, the Mental Realm, and the Spiritual Realm.
No, it's only "easy" to understand now if you read about it on coppermind, because it's:
- Certainly not that explicitly that simple from reading the books, so actually from the perspective of the readers of the players.
- Certainly not simple like that because the actual instances of these realms depend on the world you are in.
It has 16 gods, all of them have at least one magic system, and things change over time.
Not really, actuall. There are 16 shards, but not necessarily as many gods, and yes, things change. Coincidentally, how many spokes in the wheel ?
The base world building of the Cosmere's cosmology is really simple. You can explain these concepts to practically anyone and have them understand it in a single conversation.
Exactly like you can explain the prime, inner and outer planes. It will still not give you anything like the complexity that you find in the books, especially from the perspective of the readers / players.
The characters and magic inside the setting? Not so much. That's what makes the Cosmere complicated, not the actual "planes of existence" present in it.
Because you are not looking at what a "plane" is, you are just looking at principles, which are exactly as complicated as the principles of the great wheel.
That is how to create a cosmology. Have it be as simple as you can possibly make it with what you're trying to have as the base assumptions of the setting so that it's easy for people to understand, but complicate things with how the magic works, the gods and characters in the setting, and the different planets/worlds of the main physical realm.
The 5e Great Wheel is kind of the opposite of the Cosmere in this aspect. There are 25+ different planes of existence (I don't know if you count the Far Realm, Positive and Negative Energy Planes, and the Elemental Chaos). There's also probably a dozen deities that lives on every single plane of existence in this complicated cosmology. The Cosmere has a just a few planes of existence, the 5e Great Wheel has literal dozens. The Cosmere has dozens of inhabitable planets on its "Material Plane," like how Spelljammer has a ton of different settings on the Material Plane. The Cosmere has over a dozen deities and a pretty complicated timeline to keep track of. The 5e Great Wheel has hundreds of deities to keep track of (not even counting the various archfiends, archfey, archomentals, dark powers/lords from the Shadowfell, and celestial paragons that are out there) and a very complicated timeline to keep track of, too (probably more complicated than the Cosmere's).
Is the Cosmere complex? Definitely. Is it because of its Cosmology? No. Is it because of the deities, timeline, and characters in the setting? Yes.
Is the Great Wheel complex? Absolutely. Is it because of its Cosmology? Yes, as well as a lot of other aspects of the setting.
And then, complexity is a good thing, because it creates possibilities, just as Sanderson is doing by combining his 3 principles with the 16 shards and the multiple worlds. Just as the Great Wheel is doing, by looking at the possibilities of 2 directions of alignment, 3 types of planes (Prime, Inner, Outer), and a list of pantheon that can vary from 1 to a few. The principles stay simple, the implementation of combining these principles is what creates the beauty of the setting.