At this point I'm working on some of the details for how the PCs will investigate the mystery. I know murder mysteries are notoriously difficult to pull off in RPGs. My plan is to combine a skill challenge with a stack of note cards with clues on them. I don't know if this is a good idea, but my instinct is to literally mimic clue the boardgame and use the clue cards to slowly eliminate suspects, times of the murder, and motives from a list. As the 4e books extoll, i intend to have failing the challenge entail still solving the mystery, but getting beaten to the punch by a rival team of investigators and thus not getting paid (or only getting paid in part).
Sounds great
I've had great success merging skill challenges and logic puzzles. It requires a bit of forethought but it would dovetail great with your clue boardgame type setup. If you want you could also have failure provide misinformation about the target/objective.
I also need to come up with "complications" that can arise when they fail a check. I'm thinking run-ins that are about a 50-50 chance to turn into a fist fight. Maybe a group of thugs are "collecting a toll" and the party could opt to just pay them, or try to intimidate them. Another will be a pickpocket or a con-man, and they can decide weather to accept the loss or chase after them when the clock is ticking. I'd love to hear suggestions that are more specific to sigil than just a generic urban setting, or ways to make those two ideas more sigil-y.
Thugs might be Sodkillers pretending to be official guards who created some trumped up charges to bully the PCs into paying. If you DDi, Dragon #370 has an article on the Mercykillers (whom the Sodkillers 'descended' from).
Pickpocket might be a young wererat descended shifter who comes back later. If dealt with harshly, sells info about PCs to the group they're chasing. If treated kindly, offers to be their tout (guide) with his stolen wand of light and knowledge of Sigil's streets. If ignored, he might show up later getting threatened by a group of adventurers competing with PCs.
Another fun complication could be a portal mishap/trap, when suddenly a bunch of portals flare to life, maybe give the PCs a sequence/pattern code to crack within a certain time or they get teleported somewhere else in Sigil, burned/frozen on a jaunt through the planes, trapping them in a certain location for a time, or (if you feel up to it as a DM) separating the party.
Btw music I used in my old planescape campaign included Peter Gabriel, Corvus Corvax, and some of David Arkenstone's "darker" stuff... that's when we weren't listening to Live or Creed.
Also, the Wizards Vault has the audio from Player's Primer to the Outlands CD (you'll need RealPlayer):
D&D - From the Vault Presents: A Player's Primer to the Outlands CD
It's got a bit of atmospheric music and introduces some of the cant used in planescape. Ok, some of the sound effects are terrible
