Many Worlds II

We imagine that alternative histories occur in different four-dimensional space-time continua coexisting parallel to each other along a fifth dimension. This is an sf idea. Of its many practitioners, the ones discussed most often on the Poul Anderson Appreciation blog are HG Wells, Poul Anderson and SM Stirling.

Might a single conceptual framework incorporate not only alternative histories but also all the other kinds of imagined worlds? -

mythological realms, including the Nine Worlds in the Tree;
the many hereafters;
the land ruled by Oberon and Titania;
all the fictional universes;
Earths where natural selection has generated anthropomorphic animals;
the Dreaming, if we regard that as a distinct realm;
etc.

Here we move from sf into fantasy although a scientific rationale remains possible:

"...what a magnificent instrumentality the creator system was! Out of nothingness, it could bring worlds into being, evolutions, lives, ecologies, awarenesses, histories, entire timelines...They could be works of imagination - fairy-tale worlds, perhaps, where benevolent gods ruled and magic ran free."
-Poul Anderson, Genesis (New York, 2001), Part Two, V, p. 146.

How do people travel between universes? Valeria Matuchek knows theorems that enable her to arrive in the continuum that she wants or one like it. She deduces that there has to be an interuniversal nexus and thus enters the Old Phoenix which is in a pocket universe. Meanwhile, in Neil Gaiman's The Sandman, the Inn of the Worlds' End is maintained in existence by the continual ending of worlds.

Returning to the infinite but bounded Dreaming, Morpheus spirals past way stations on the fringes of nightmare, then charts a course nightward to the Gates of Horn and Ivory. (See here.) Later, traveling from the Dreaming to Hell, he passes through the cold wind of the uncreated wastes.

Maybe there should have been a collaboration between Poul Anderson and Neil Gaiman?