Indo-European planes

A Proto-Indo-European (PIE) campaign setting would have a much simpler cosmology compared to D&D’s typical bazillion planes. The PIE planes would consist of only a handful of planes, if they can even be called planes at all. We have the primordial cosmos that existed before the world, the world itself, and the underworld. There’s no heaven or hell, that’s a Christian thing. There’s no elemental planes, because it’s unnecessary.

The primordial cosmos has few to no details associated with it, but it’s probably where creatures of chaos come from.

The world consists of the sky, the earth, and the ocean. The earth is a flat disc bordered by the ocean. The sky is a dome upon which the celestial bodies travel. This is where the adventures happen.

The underworld exists below the earth. It’s inhabited by the shades of the dead and possibly by monsters of various stripes.

These cannot necessarily be said to be planes, since you can potentially travel between them by walking. Plane shifting spells or transitive planes aren’t needed.