I disagree. The notion that you can't have other planes in the Great Wheel is- well, it doesn't hold true in my experience. Just because there are so many planes of a certain type that we know of doesn't mean there aren't others, or haven't been others.
The para-elemental planes are set up differently in the 1e Deities and Demigods, for example; we have Vapor, Heat, Ice, and Dust as the planes between the elemental planes. Oriental Adventures implies additional elemental planes of Metal and Wood (one of which was later developed in the 3e MotP as an optional plane). In 2e, which was still married to the Great Wheel, we saw the addition of the Far Realm in the Illithiad. There is no reason you can't add more; in fact, if you look at the logic of the Outer Planes that don't correspond to a pure alignment, such as Pandemonium being a plane of CE/CN, there is an implication that there are missing planes between Concordant Opposition/the Outlands and each of the four planes of Limbo, Nirvana/Mechanus, Elysium, and Hades.
Heck, the text of the shocker suggests an unknown plane of Electromagnetism. Yes, this is later retconned into being either a demiplane or the quasiplane of Lightning, but there's no reason it has to be so.