Multiplayer/Co-Op & Frictional Games? NO!!!

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Well...I think Journey manages to combine co-op with (almost) perfect immersion. But thats an entirely different style of game of course and in no way horror.
Although I found it pretty interesting, that the parts in Journey where enemies were present ALMOST suceeded in being kinda scary. Sneaking past those things with my buddy was really tense the first time I played it. And when he got hit by one of those beasts, I was really worried for a moment, that he might be dead. Especially when he didn't get up right away, but continued to lie there motionless I immediately rushed to his side and chirped all panicky ^^

I think that sort of anonymous, no-communication multiplayer has great potential and maybe you could even make a horror game with that. But it would need to be vastly different from everything thats on the market right now.
The strenghth of multiplayer in horror, imo, could lie in the fact that as social animals, human emotions can be increased by the emotional display of other people around them. Imagine walking home at night with a really paranoid friend who jumps at every shadow and whose body language tells of his fear - after a while you start to get nervous too. Amnesia actually already does this sort of - when you hear the protagonist breathing rapidly or gasping at a sudden jumpscare it heightens the tension because you feel with him.