In my own campaign, I’ve replaced the gods with worship of devils (representing lawful evil) and saints (representing lawful good). Chaos (in the form of demons and elves) is the big threat in the world.
And of course, being an evil DM, the first few quests the PCs came across were offered by the local priests of the Nine Hells, complete with contracts signed in blood and a helpful (rather, annoying) imp sent with them to keep tabs on their actions.
The fun part of this approach is that it bakes a lot of fun roleplay and intrigue into the campaign. To be honest, the priest of the Nine Hells they talked to didn’t act all that much different from any number of vanilla NPCs who need the characters to do something for him.
The flavor (sinister tiefling! infernal contracts! devil worshipers!) turned a simple fetch quest into a really tense, interesting decision point for the entire party.
My rule of thumb is pretty simple: If the players have at least one argument about whether they should finish a mission or quest as planned, you’re doing a good job of bringing some interesting RP to your campaign.
A few easy ways to do that:
* As the game progresses, the PCs learn information that make completing the quest seem like a possibly bad idea. If it’s an obvious bad idea from the start, the obvious play is to turn it down (though really engaged players might try to undermine a faction from within and take the job).
* Keep information in shades of grey. The priests of Hell are evil, but they also keep demons from overrunning the city. They’re evil, but more “betray you and take your stuff” scheming evil rather than “flay you alive and eat your skin” crazy evil.
* Keep motivations obvious, but plans opaque. The PCs know the priests of Hell are scheming, greedy, and selfish, so giving them a piece of the Codex of Infinite Planes *might* be a bad idea. It might be bad, rather than is clearly bad, because they don’t exactly know why the priests want it. They might want it for selfish reasons, or they might need it to stop a demonic incursion.