I like the idea that different fictional worlds are ‘really’ the same place: for example that Sherlock Holmes’ London is the same place that got invaded in The War of the Worlds, by Martians who interacted with Edgar Rice Burroughs’ Martians. R’lyeh is really Atlantis after it sank, and so on.
I was thinking of writing a ‘pseudo-history’ (similar to Last and First Men, but probably shorter) which unified as many science fiction, fantasy and occult ideas as possible. The overall idea would be that people in our universe, who thought they were making ideas up or receiving divine inspiration, were really getting visions of this other universe.
Please let me know what you think of this idea.