I quite like the mix up idea. The players are monster hunters, hunting a monster shockingly enough. One monster, but there are multiple players. At the start the monster would choose their type, and depending on type, would get an opportunity to place their items (if any) on the map before the players arrive. For a vampire this may be a coffin, which would have to be destroyed, or for a demon an altar, which would have to be consecrated. You could have a werewolf or something which regenerates rapidly out of combat.
Allowing some monsters to have abilities would also be great. Perhaps the vampire isn't all that much stronger than a player attackwise, but can build traps to defend its coffin. A player could stumble upon the coffin, notice the traps, and instead collapse the roof on it with an explosive charge of sorts.
The main downside to this would be getting players clustering together and camping, but I think the ideal solution to this would be a fear mechanism. Perhaps light items are scattered around the map, and slowly burn down. When in darkness, fear kicks in, and the players start hallucinating - perhaps they hear footsteps, or random blurs dash across their vision. If it was really bad their hands would start shaking, ruining their aim (which would work somewhat better in a modern based game, but ah well.) Thus they're encouraged to move.
.....Anyway, idea dump over
I like those ideas. As the game progresses, I'll definitely be referring back to this!
Also, would anybody be interested in playing the super-early,-earlier-than-Warden-alpha sometime soon? It's pretty basic, but you can set traps (explosives, barbed wire, and alarms), bar doors, die horribly, and (if you set up traps right) kill the monster.