In putting together a cosmological chart for my Anethemalon campaign, I found a couple of things looking like add ons in 3.5 D&D and 3.5 OGL cosmology.
The first thing that I took note of was the Concordant Opposition. This appears to be an outer plane to place the powers of neutrality or balance. My take here is that the balance is always shifting. It is not a thing of itself. It has its champions and they play out their cosmic shifting actions on the stage of the Prime Material Plane, the home of neutrality. This is the place between all of the outer alignment planes (or celestial/spiritual planes) that act against one another and where their shifting is felt. Here too the Positive and Negative Material planes mix along with the Elemental Planes to create reality. The mixing creates the uneven neutrality that is the Prime Material.
The other thing that really seemed unnecessary to me is the Shadow Plane. This is supposed to be some negative plane of the dead or something. With the ethereal plane being the home of ghosts and other disembodied creatures, how is the Shadow Plane required? In my thinking the ethereal plane is created from the interaction of the positive and negative material planes. Not all of the ethereal is of the disembodied undead, but the negative parts attract and trap them on their way to the spiritual planes. Also the ethereal is what prevents the inner planes (or as I call them the temporal planes) from collapsing. The ethereal creates enough resistance, due to the repulsion of the positive and negative material planes, to prevent the elemental planes from negating each other. Just enough of everything is allowed to seep in, mix and create the universe.
As noted above, the Prime Material is a mixture and as such is the only plane without an opposite, thus creating a stronger case for the redundancy of the Shadow Plane.
Neither Shadow or Concordant Opposition are in the AD&D Cosmos (Prior to Jeff Grubb's Manual of the Planes) It looks like Gygax actually thought about how this whole thing might work. I won't be including either.