I would see the LARP like this:
Story-
Parasol Inc, has developed a wonder cure that will cure any disease on the planet. Little did Parasol Inc know that this cure would mutate in human hosts after a year incubation period. Now it has become a highly contagious disease that has run rampant through the entire world. 75% of the worlds population has been infected. Only 25% were immune to it. However, this disease turns infected humans into flesh eating zombies! It is up to you, the remaining human population, to find a cure for this horrible disease and to survive!
Races-
Human and Zombie
Classes-
I see this system being based on a "Merit/Flaw" system in which players (Zombie and Human) purchase skills that would aid and hinder their characters. On top of that add a simple system that is easily translated to newer players, yet still complex enough that players don't get bored with the mechanics. Therefor, there would be no "Class System" but rather a character makes a concept and can purchase skill accordingly.
Skills-
I would throw in a skill set for both Zombies and Humans. Include future skills like Chemistry, Engineering, Gun Use, Medicine, Hacking and stuff like that. Also allow for zombies to get useful skills such as "regenerates unless killed by decapitation" or "Blood is like acid" and skills like that.
Magic?-
I most likely wouldn't use magic at all. Instead replace it with more unique production skills. Acids instead of elemental based damage. Armor that protects against attacks instead of "Protection spells". Stuff like that.
Examples:
Joe decides he wants to play a zombie for an overnight game. He decides that for his flaws he wants to be "Slow Moving" (Cannot run) and "Dumb as a Rock" (Cannot open doors or use any sort of technology). But for Merits he takes "Tough as Nails" (Bonus Health) and "Fists of Fury" (Bonus Damage with Claws or Fists). For skills he takes a lot of damage for his claws and some defenses so he won't drop like meteor from the sky. Now Joe puts on his costume and is ready for the game!
Erica loves playing zombies! But this weekend she wants to see what it is like to be hunted. She isn't great at melee combat so she decides to make a "Scientist" type character. She picks up merits like "Smarter than a 5th Grader" (Allows this character to use more advanced items and technology) and "Prodigy, Engineering" (Bonus to Engineering skill). She also decides that her flaws will be "Yellow Belly" (Fear effects last longer), "Weak Stomach" (Nausea like effects last longer), "Old War Wound" (Lower Health than normal characters). Erica now purchases skills like Engineering, Computer Hacking, Chemistry and Medium Gun usage. During logistics she creates a bunch of items that will help her and her friends out... (Items like Health Kits, a Rail Gun for a friend, and some good ol fashioned Dynamite.) Now Erica is ready to rock those Zombies!
*shrugs*
Paul Iverson