The Mist as a psychological survival horror? That sounds really cool. I pretty much was shunned by the community for wanting to use the IP so I abandoned my effort. But, with a change of theme, and different monsters, I think core elements of a small group of players trapped in an area under attack by monsters, demons, aliens, or whatever, could still be fun. Co-op would be a must.
I’m not certain about the trend of single-player games falling out of favor for co-op, but, I personally prefer co-ops because you can design it with the same gameplay as single player game, and allow players to bring along a few friends to share the experience. I only buy Console games that Co-op. All of my games are designed as Co-Ops and include both local Co-op (Splitscreen) and remote Co-op support.
I’m actively looking for ways to utilize co-op in the game mechanics explicitly and implicitly. For example, in a stealth game, a second player, can act as remote viewer or scout in spectator mode, providing info for the first player to use to navigate more efficiently. How about controlling a single character with two or more players? IE: Giant Robot or a Protagonist who suffers Multiple Personality Disorders.