Some representative sources of ideas:
All sorts of stories can rub shoulders in the Commonwealth of Letters, a la Silverlock (John Meyers Meyers).
High tech works in some neighborhoods, magic in others, and swords everywhere in Cynosure (John Gaunt/ Grimjack's hometown, from 1980s comicbooks).
Tekumel (world of the Empire of the Petal Throne game) is both on another planet and in the very distant future. The human cultures are slightly reminiscent of pre-Columbian America, also of ancient India. See also Dancers at the End of Time, Dying Earth, Majipoor, Urth of the New Sun, Long Afternoon of Earth (a.k.a. Hothouse), Instrumentality of Mankind
There's the old time jumble, a la October the First is Too Late (Sir Fred Hoyle) -- somewhat evoked in the game Torg.
Everybody who ever lived can live again (and again) in a world of resurrection, a la Farmer's Riverworld.
Or they could be Heroes in Hell.
There's always a multiplicity of worlds, a la Farmer's World of Tiers, Zelazney's Amber, or Tom Moldvay's Lords of Creation game (which includes a brief write-up for a world based on the visionary poems of William Blake).
There can be adventures inside a sentient world, as in John Varley's Titan/Wizard/Demon.
There's room for many (billions of?) Earths on Niven's Ringworld.