I am not an expert on the planes of Magic, but my understanding is that individual M:tG worlds can have their own local cosmologies with subplanes (Theros being an example I think). Assuming that I have not go the WEOTS, it makes more sense to me that the the Great Wheel be another such local cosmology within the greater whole rather than the other way around. With TGW being a plane in the Magic sense, and its various components the subplanes.
Also, if there anyone is interested in the Wall of the Faithless, there is a
lively debate going on about it on
RPGnet right now....
_
glass.