"Zombies are easy. They're stupid and they can't run. Werewolves, on the other hand.."
We're going to take the existing well-tested Outbreak format (nerf guns versus zombies, in the dark on Hampstead Heath), and adapt it to a new style.
This game will run in the afternoon, so it'll still be light. That means it's safe to have chases, which means that the enemy will be fast-moving wolfpacks instead of slow, shambling zombies. The nerf guns will work exactly as well as they did before (lethal, if you can find ammunition and figure out how to load it), but a new "howl" mechanic means that any of your friends might suddenly turn into a werewolf and come for you.
This is the game design session where we work out how it's going to run!