Small multiverses in fiction.

[Seppl]

Star Trek has exactly one parallel universe, the mirror dimension where everyone is their evil (or good) counterpart. Which highlights one of the potential problems with the idea: The multiversum plot device is usually handwaved with the multiversum interpretation of quantum mechanics, giving the authors a convenient excuse for visiting a what-if-scenario. That handwaving requires both, infinite universes and that the differences be small (at least at the point of divergence). A limited number of universes with highly specific rules needs an explanation of why this specific set of universes exists because it looks too much like intelligent design (which, of course, it is, because the author constructed it that way ). That might not mesh well with settings that go for a more scientific feel in their worldbuilding. Star Trek has just enough god-like beings in the setting that might explain why all dimensional travel goes to the one parallel universe that evolved exactly the way to contain an evil twin for everyone currently alive. But then again, the inclusion of literal gods always felt strange in Star Trek.