While this may seem like an utterly vanilla cosmology, in fact it suggests a view less like the "Great Wheel" and more like the medieval view of the system of celestial spheres. In religion before the modern epoch, other planes were supposed to be physically located on other planets. The astral plane, which first comes into D&D through the Astral Spell in Greyhawk, is literally the star-filled void between these spheres. If we begin numbering planes at 1 for earth, the pictured diagram of celestial spheres indeed brings us out to a 12th sphere - so that the ones Magic-Users do not contact are the Earth and the Moon. This view of the world ultimately stems from Aristotle in De Cælo, or On the Heavens.
I'll have more to say about the numbered planes in the future, but what is memorable about them is their directionality. Since the highest plane contains the most truthful entities, but also the greatest chance of temporary insanity, it makes sense that each plane should be further removed from the mundane reality, stranger and more surreal. But as the astronomical chart above shows, there is a plane "2" that is missing from both our world and Contact Higher Plane: the Moon.
Great Moon Hoax of 1835. It's a story of a mythical telescope which revealed the kind of civilization that Victorian people expected to find on the Moon, and it didn't disappoint. The centerpiece were the "vespertillo-homo," lunar people, who were short, about 4' tall, furry, and possessed of bat-like wings. They had taller, whiter cousins and lived in relative harmony with the other species. This is a moon covered with crystals, flowers and volcanoes, where there is a great temple of sapphire made to look as if it is on fire; there are buffalo-like creatures, blue goats with one horn, and even tail-less beavers that walk on two legs and live in huts. Tell me that isn't a place that is just aching for weird encounters! A portal, a "moon pool"? Who cares, it's practically made for adventure. So yes, you can anticipate a full complement of authentic lunar creatures in the bestiary. Maybe even an adventure.
Astral Spell has one further tantalizing possibility, which also ties in with our directionality. A character whose body dies while in astral form will find themselves in "the lowest hell." And yes, we will be going there.