Fictions And Fables

A subset of the parallel universes idea is that characters who are fictional in one universe are real in another. Thus, Robert Heinlein's Rhysling, the Blind Singer of the Spaceways, Poul Anderson's Nicholas van Rijn and inhabitants of the universe where Shakespeare's characters coexist can all visit the inter-universal inn, the Old Phoenix. Thus also, in Bill Willingham's Fables, fairy tale characters are exiled to "mundy" (mundane) New York when a multiversal empire conquers their homelands.

Alan Moore's The League Of Extraordinary Gentlemen has every literary and fictional character coexisting on a single Earth but that gets a bit crowded.