Hi everyone -
I was involved in the initial plotting, so do not speak for the fine team at Cyanides, but I can share some information that is already out there:
Call of Cthulhu is a single player investigation game. To approximate the feel of a team of investigators, the game has a companion system; you recruit allies and can then send them off to do things. You can read more about it in this preview on PC Games N (bonus, the author is a CoC RPG player):
And, here's the first preview, from Vice:
Blackstone, your idea is cool. But, from my experience of 20 years in videogames (and, importantly, my opinion, not that of Focus or Cyanides), adding full multiplayer requires considerable budget: new characters and animations, voice acting, code to make it all work, puzzles that make use of multiple players, etc.
Traditionally these games are single player experiences, and it seems that folks are comfortable with that. Where we do see popular and best selling horror multiplayer games they seem to be FPS (eg Left 4 Dead), because shooting and action is a lot more straight forwards to make than multiplayer investigation. (I don't take anything away from the skill of Valve or just how awesome Left 4 Dead is, it's just that when you make a videogame it's nice to have a clear focus). Left 4 Dead also perfectly suits the quick drop-in-and-play-culture of online games, whereas an investigatorial multiplayer would work best with a dedicated group of friends who all go through the mystery at the same time.
Again, I think that it would be really awesome to play a game as you describe it, but once you add the technical issues and the smaller market share, it might be hard to make a game to the right quality level and still make money. Perhaps this is something that we will see evolve in the indie space, where people are free to develop ideas without the constraint of next-gen graphics.
Luckily, we will always have regular CoC for the perfect multiplayer experience!