Sounds like my current Kingmaker campaign: set in a particular area of the Golarian world using 5E rules and a Celtic pantheon and calendar. The rest of the world might exist in reference (e.g. many escaped slaves from the demon-pact nation of Cheliax settle in the area you're now in), but I don't need the rest of the world right now or perhaps ever. The PCs have a lot of liberty to create NPCs during character creation.I've used a lot of, "Collaborative homebrew based on published work".
Basically, this thread brings up the question: What does it mean to "use" a setting?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.