From the film I gathered the core of the story was of a ‘The monsters are the people inside the building’ type. For singleplayer I could picture it working as a heavily story-driven game like Mass Effect, where you have to get other people who are trapped to join your ‘side’, with occasional missions outside of the building to get supplies. But as a co-op game, I get the L4D influence - that would also work pretty well.
By ‘falling out of favour’, I meant that a lot (if not most) of the popular indie titles recently are either open world single-player or co-op, there’s fewer ‘true singleplayer’ games reaching notoriety with the exception of horror games. But I prefer Co-Op as well, which is why we’re putting emphasis on it for Acedia.
The stealth game mechanic you speak of (i.e. asymmetrical co-op) is actually being done by an old friend of mine. It’s definitely a better stealth mechanic than, say, Payday 2 where 3 players end up waiting for 1 player to complete the ‘stealth’ component of the mission.