maybe it could be more like an resident evil outbreak type game. where you start with 4 survivors and you have to solve puzzles to get to the end. but you have to survive zombies and other mutants to get to the end. if a player becomes a zombie he has to stop the players.
or maybe it's pvp where some players are zombies and some players are humans. like L4D. and humans have objective to win while zombies have the objective to stop humans and convert them. last human standing loses and humans converted to zombies are winners. of course winning/losing doesn't matter. it's how you play the game that matters.