Although the game is dominated by zombies, would it hurt to have an extremely rare monster (8 feet tall and scary as ***) lurking deep within the forests? I'm not talking about turning the game into something like S.T.A.L.K.E.R. - keep it primarily a zombie game with a bonus giant mutant zombie....thing! Throw 1 or 2 into the game every week or month (if it dies, it won't respawn until then), make it extremely hard to find (it could attempt to hide from players and charge when attacked or approached -taking more than 3 players or xxx number of rounds to kill it). Alternatively, you could have it aggressively hunt and attack players upon sight as soon as they wander into its part of the woods (within 40 meters unless the player is very sneaky). The creature should keep moving, remaining well hidden in the treeline and only move from one large patch of forest to the other (or could approach farms) and cross fields at night -just so players can't track it down if they get killed by it. It could be something like a scary mutated zombie or a sascquach monster; however placing a 'Ski Free'-like (super fast) yeti in the game is fun idea I've been entertaining lately (if so, make sure it rubs its belly and jumps up and down after eating you! : P) http://www.youtube.com/watch?feature=player_detailpage&v=aACwPlrGIGg#t=9s