GX.Sigma
Adventurer
I think the answer implied by the structure of the cosmology is that the inner planes came first (elements), then the ethereal (creation), then the material (existence), then the astral (thought), then the outer planes (belief).One thing to consider is how the various outsiders relate in terms of age to the planes and to mortals (and potentially mortal belief). I'm in favor of creating the planes and then afterwards populating them with creatures that make sense for the plane and for that cosmology's history.
Within the Great Wheel a lot of this is of course some very much after-the-fact rationalization for scattered, create cool stuff as we go, without a unified vision for all that time method of world creation from 1e to 2e to 3e etc. But I think it's important to take into account. If a particular outsider race predates mortals, do they necessarily need to be linked to any particular concept that easily makes sense to mortal conceptions? Or can you be a little more vague about it?
Of course, there's also the idea that the outer planes are self-sustaining (unless you imagine that the powers are holding them together, so they are indirectly sustained by human belief), and the implication that the planes were created (hence the astral plane, which nobody was really meant to see).
Hey, it wouldn't be Planescape if these questions had easy answers.