I bet you didn't know that you can go from the Wilderlands to Greyhawk via Minaria. The map below almost sort of works, if you assume Rhombume (the pink realm at the bottom of Minaria map) is a peninsula and that the Waste of Va-Ka-Ka and the Land of Black Ice are somehow the same.
That's the Known World of Basic/Expert D&D tacked on to the bottom. Since you have to go through the jungle or desert to get to it from Greyhawk, maybe it could serve as the Flanaess equivalent to the Kingdom of Prester John.
I think Ed of Esoteric Murmurs is right, the best explanation for this arrangement would be that they are all part of the setting of The Fantasy Trip, Cidri. The canonical description of Cidri suggests that it is a flat earth of vast size, but I prefer to imagine it as a hypersphere. That would give it a crapload more surface area than an ordinary planet, but it could still look like a sphere in 3 dimensional space and it could still orbit a star (sunrise and sunset over a large plane seems tricky to me). But it would look radically weird from space, as whole new continents slide across the surface for days on end.
[map] An architect's bad design
6 hours ago