This is a very rough explanation of an idea I had this morning. It takes obvious inspiration from both Clue and Mafia. I'm not going to elaborate on the actual setting I have in mind for the game, since it isn't relevant to the overall concept.
Clueless: A mystery, wrapped in an enigma, crammed into an RTD
Y'all wake up one day to find, lo and behold, somebody, we'll call him Mister Body, has been murdered. He was one of you, and now he's a corpse. One (or maybe more) among you is/are secretly playing as the murderer(s). Somebody, we'll call him the butler, has locked you all in. Or maybe you physically can't leave, depending on the actual setting. Regardless, you don't get out unless you take out the killer(s) before s/he eliminates you all.
Play begins in a centralized meeting room. This is the only well lit, open space on the game map; therefore, actions that happen within this room must be publicly posted on the main thread. However, the vast majority of actions will (presumably) take place throughout the rest of the building, as the players will be expected to investigate the circumstances of Body's death; therefore, most of the game will take place via PM's between myself and the players. They send me their actions, I send them their results and describe their surroundings. No player-player PM's allowed.
The players, if they have any sense whatsoever, will agree to return to the relative safety of the meeting room at certain (in-game) time intervals, to discuss their evidence. Of course, as this is still a roleplaying game, players may very well choose not to show up at all. Whether said players are simply hiding in a cupboard somewhere, in hopes of waiting things out, or if they happened to run into the killer, is up to the players to determine; there's no good way for them to divine the status of a missing player without finding a (possibly dead) body.
In order to encourage cooperation, I will be sure to scatter weapons (and other useful items) all around the map. Probably traps too. Certain players (the killer(s) in particular) may happen to start with such items in their inventory, as well.
Sound vaguely interesting?