Concept: "Last Voyage of the Demeter" RP or LARP where Dracula isn't the only possible danger on the ship. Instead, the monsters are chosen at random -- like, every player draws "roles" from a deck of cards or whatever, if it's not a video game -- and the dangers range from mundane-but-abnormally-skilled murderers and assassins to Classic Horror Movie Monsters to devils and freaking Outer Gods. The actual monster-monsters all have different ways of killing them (i.e. silver for werewolves, stake through the heart for vampires, etc), and if you use the wrong one it might be actually counterproductive -- you shouldn't throw a shapeshifted Deep One overboard, for example.
But also, there could be a nonzero chance of having multiple monsters. Bonus points if the monsters don't know each other's identities, like a werewolf will try to put the bite on this one passenger and she turns out to be Actual Satan in disguise. If the crew and passengers are lucky (insofar as having multiple monsters is already unlucky), the monsters will be too busy messing with each other to go after many victims and they'll make it too obvious who and what they are. If they're not lucky, the monsters will cooperate to the extent that they can get away with doing so. If they're really unlucky, the monsters will be friendly with each other.
