I grabbed a bit from previous modules we ran and from the Mafia parlor game (or Spiders, as we have it here). A few goals were:
-Have a clearly defined winner. Other ones we played it was a bit anticlimactic in that they felt like the players' actions had little effect on the result.
-Make trust necessary for success, but also dangerous
-Add a high risk/reward element with secrets
-Add some structure, because previous games had a few people losing focus
Players come in knowing their characters entirely - what they can tell people and what they shouldn't. However, some characters know secrets about others that they bring up in conversation to get the other talking. This is set up on Round Cards. There are three rounds and each has a Round Card. The card explains who you need to approach and what key phrase you need to use to get them to divulge information. It also says what information you need to divulge when someone approaches you with a specific key phrase. So during the social part of the round, the players MUST talk to at least two other people, and may talk to as many others as they like, saying to them what they like.
The players also start with a fixed number of credits. I'm playing the role of an Information Broker, holding one additional piece of important information on each player. Broker info is unique in that it can be either good or bad, depending on the player, so there is incentive to control your own as much as others'. After the social part of the round, any player can open bidding on any player's information - including their own. Bids are secret, so they hide a number of credits in their hands and all players reveal their hands at once. All bids, whether they win or lose, are final, and the Broker keeps those credits.
Once players have the information, they can choose to do with it as they wish. If it helps to clear their name, they can let others know. If it helps condemn someone, or if it's not entirely clear which, they can accept bribes in credits from other players, who then are free to keep it to themselves or say it aloud as they see fit. At the end of each round the Broker presents to everyone one new piece of evidence in the case that will point toward the murderer.
At the end of three rounds, a simple majority determines which player gets brought to the authorities - using the acquired information from the broker, social rounds, and evidence, they have to convince each other to turn someone in. If the murderer is not chosen, they win. If the murderer is chosen, the winner is the player who voted for the correct person and has the highest remaining number of credits.
I'm really looking for simplicity here, but with the potential for enough strategy that it's still interesting. Any feedback is appreciated!