In the spirit of open design, I'm about to post the two existing systems for
our (that is,
drcpunk,
ladymondegreen,
batyatoon,
akawil, and my own) long-in-development, 60+ person full-weekend theatre style LARP: "Dark of the Moon".
A bit of context may be required to understand the design goals here:
A traditional theater style game (as this is) uses GM-designed characters played over a limited time game in a single (usually) session. The characters come with pre-written relationships, histories, and backgrounds, but are not scripted -- instead, they are entirely within the players' control once the game starts. Morover, there are generally few characters beyond those played by the players, and the most important characters (and all important sides of any issue) will be player characters.
In a good game, there is no pre-determined course of events -- there may be some pre-set events at various points to help shape the narrative (for example, Dark of the Moon, as a psuedo-victorian setting, will have a formal ball at some point), and the GMs may intervene to help control pacing and try to make sure the game's material covers the entire time alotted, neither coming to a grand conclusion within the first 25% of the game nor petering out before most of the events can come to a climax, but exactly what will happen is at the players' discression, the GMs primarily acting as facilitators and judges.
Often, characters will have secrets -- these may be secret to most of the other characters, or to all but a few, or may even be secret to the character in question in some fashion. How these secrets are revealed (and while many games allow secrets to not end up being revealed by the end of the game, I feel it's important that they -are- revealed for the most part, and this is reflected in my mechanics) impacts and changes how the game plays out, often in very extereme ways.
Ok, that should be enough context -- if you have questions on how this whole concept works, rather than any (either) of my mechanics, post them here.