TOEE was originated by Gygax and crew, as was all of Oerth. I'm not real familiar with Oerth, but many of the artifacts going back to the original DMG are full of Oerth history -- Vecna was from Oerth, for instance.
And, Planescape was not the first way that planes could be linked. Manual of the Planes set out much of what you could get from the original DMG and MM's. Ravenloft is always fun -- and even has a land dominated by Vecna. Let's not forget Spelljammer.
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