When I woke up, I thought: That's not a bad idea. I should write it down.
I've refined it a little while composing it, but this is basically the idea as it formed in my dream.
This is a generic LARP system. It's designed to be very simple, and can be used to execute pretty much any kind of story.
The central premise is that the players have all the roles present in the plot. If there's a villain or murderer, that character is played by one of the PCs.
The second premise is that the players will not know what type of story they're in, unless their character does. Rather than telling the players from the outset 'This is a murder mystery' or 'the story is about finding the stolen 'Eternity Diamond', or 'it's about rescuing the kidnapped prince,' the players will only get information germaine to their character. So they won't know it's a murder mystery unless they're playing the killer who's hiding the body.
The final, key premise, is that the LARP is not about accomplishing your characters' goals, but about playing your character well. Accomplishing your goals may or may not please the other players, but if everyone plays their characters well, the game should be fun, even if you 'fail'.
Here's the set up.
Each player gets a few pages briefing them on
1) What their character is like
2) What they know about the other characters in the game (including such things as whether or not their character likes another character, for example.)
3) Character goals. These can be simple things like 'Get player C to agree to dinner and a movie,' or complicated things like 'Get away with murder.' Every character should have several different goals.
4) Character abilities. These can be anything: the ability to fix inanimate objects, or defend against an attack, or assassinate other characters. Abilities should generally be germaine to the plot. They should also be simple and easily carried out -- not a lot of die-rolling or paper-rock-scissors playing. You succeed or fail, and move on.
5) Icebreakers. This should be a simple list of conversational gambits or other reasons to engage particular other players in conversation. The idea here is to give people a 'starter', so that they don't stand around wondering what to do next.
They also get a list of:
1) Character quirks. These are roleplay traits, and can be simple things like biting one's nails, saying 'uh' a lot, or smiling constantly, or has a British accent. Or they can be important things like 'Wring hands whenever you lie (and only when you lie)' Or 'hates player D because he tried to date your sister, and so you always say bad things about him.'
2) List of things they know and should share. These may be simple things. For example, Player A dislikes player B and is annoyed by his habit of wringing his hands, and Player A should share this with players C through F. 'Things to share' may be meant to be shared with all other PCs, or only with certain ones.
To help identify players as their characters, character portraits should be printed next to each name on the briefing sheet, and each player should wear the character portrait of their PC. (This could be on a badge, or hung around the neck, or whatever.
The game itself can play out in any number of ways. But to facilitate the key premise -- that you 'win' by playing your PC well -- all players get evaluated in a post-mortem, after the game is over.
The post-mortem scores players on whether or not they played out the items on the second list: how well they stuck to their quirks, and shared the things they were supposed to share. A multiplier is assigned for 'sharing naturally' -- ie, you don't want to blurt out your information randomly, but insert it into the course of conversation.
To handle the post mortem, each player gets a copy of the each other's list, and checks off whether or not they observed the PC's quirks, or learned what they were supposed to learn from that PC. They also check off whether or not it seemed 'natural.'
The GM can score everyone based on these tallies, and declare a 'winner'. But the real motive behind the post-mortem should be less winning or losing, and more about how to improve so that you'll be better able to participate next time.