I don’t know why it is that I come up with ideas for campaigns that I will probably never run. I suppose that if I put them up here, someone might get some use out of them. In any event, in this edition of Campaign Ideals I’ll Probably Never Use, I give you a D&D campaign:
Concept: The characters work for a private investigation firm in Sigil.
Explanation: There isn’t much more needed than the concept. The Planescape setting does most of the work for you. Toss in a mysterious owner for the P.I. firm and blatantly steal plot elements and tone from Noir fiction.
Rule variants: You’d probably want to keep this fairly combat-light and skill-heavy. Extra skill points and/or a few free class skills wouldn’t be out of the question. I’d suggest starting PCs at about 3rd-4th level and keeping advancement slow. Don’t give experience points for killing things. That isn’t the goal. XP should be given out for uncovering clues, solving problems, and resolving cases.
Variations on the concept: For a slightly more traditional variation, try Shadowrun in Sigil. Factions replace corporations. The PCs become troubleshooters and theives for hire, specializing in tricky jobs. Some of these could run a lot like traditional dungeon trap-fests. PCs would have to depend on ingenuity and clever use of resources.