In the core assumptions of pretty much every fantasy system that isn’t brutally low-fantasy, there tends to be a premise that there’s at least one god, or perhaps a full pantheon of gods, who are behind the world as movers and shakers on a level that mortals can only hold a vague hope of one day brushing the lower echelons of. A few have usurped this to some degree – in the D20 system, the settings of Eberron and Dark Sun both did away with provable active gods – but this tends to be the default state of fantasy games.
There’s nothing wrong with it in general, and quite a lot of exciting stories can be told where such being are either the source (an ancient evil god seeks to return, a new evil god is going to be born, a good god or alliance of good gods deputizes some mortals to go fight the minions of some evil gods or other horrors, and so on) or only involved as much as the player with a priestly character calls the god’s name when calling on the god’s power.
Still – there’s something to be said for worlds where there’s absolutely no divinity; not even the vague “Well, maybe, because these priests are getting power from somewhere, right?” of Eberron. There are almost certainly going to still be religions, but there’s nothing to back them up. If the setting has things like angels and demons, they’re just another kind of life – possibly from another universe, possibly lurking in unusual places and using the beliefs of mortals to their advantage.
This premise opens up a whole host of new possible stories. If there are no gods, and the players all know it even though faith and religion are still things in the world, how do they react when two different priests of the same faith beg them to help deal with the other as a blasphemer and traitor (possibly over some minute difference of belief that seems absurd to anyone outside the faith)? What about if they come across a village that worships the statue in their shrine as the avatar of a god, and it seems to grant them miracles (perhaps a restless ghost, seeking to incite the villagers to finish what tasks it had, is behind it. Perhaps an angel or demon has decided to ‘prove’ the god is real by working miraculous effects in the name of it. Perhaps the villagers themselves are unwittingly possessed of some unusual power that collectively lets them bend reality together, and this is how it manifests.)?
It also opens up the question of players deciding to take advantage of the situation. Charlatans and miracle-peddlers who intend to never come back could easily have a campaign built around their misadventures, with angry former customers hunting after them as they go from town to town, leaving emptied coffers and broken dreams behind, along with the occasional genuine bit of good. Perhaps the players are the charlatans, always trying to stay a step ahead of their vengeful former customers, or perhaps they’re a band of people cheated by charlatans hunting after them.
All in all, a godless world poses quite a lot of unusual ideas and plot hooks that could provide a wide range of fodder for stories and plots. One-shots can be built around scenarios like the ghost in the statue or two competing bands of false priests arriving in a town, while a more complete camapign can build on any number of ideas, including a ‘war in heaven’ between angels and devils, with some renegade factions trying to reveal the truth to the mortals.
Certainly seems like something worth giving a try.