Well this is a basic idea for a mystery game. I just had it, but I would like to hear the opinion of other players.
The GM creates a mystery (can be a kidnap, a murder or any crazy situation you like. Even a supernatural mystery).
Then players join the game.
After that the GM designate a culprit or culprits that won't be a character played by it (as it would be too obvious), So it PM one of the players to be the culprit, give to it free act, yet it order certain tasks that must be done in the game.
And well other players had to
1) Find the culprit.
2) Try to be safe.
Regarding point 1, it would be like this characters can suspect of someone and expose his/her arguments, however this imply the risk to be targeted, and also would be ignored if it really doesn't give a good argument or if it's contradicted
Now where is the interesting part?
Characters's thoughts are allowed to be a lie. In other words player can write fake thoughts for their characters. This of course including the culprit. Making it hard for the readers to solve the case.
For example the very culprit could just write a thought for his character that would make others think that it's innocent. And in truth is just a lie of the narration, while it's nt something that the culprit would had thought.
To make it more confusing, not only the culprit but players can thought-lie. And also the GM can force players to thought-lie via PM
So waht do you say?