The problem here is there doesn't seem to be any consensus over what it means for a game to be horror to begin with.
Everyone is saying that pacing in a sandbox game is difficult to manipulate... but that makes it sound like we're talking about jump scares. What about atmosphere? A lot of atmosphere is created passively. Art design, background music and noises,
ideas.
Here are a couple of examples off the top of my head:
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AVP2. Gives the player a motion detector that will go off
all the time. Works both as a jump scare (oh shit they're coming for me RIGHT NOW) AND as atmosphere (oh shit I have no idea what's making that noise, it could be a broken door or it could be an evil alien that wants to eat me).
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Dishonored SPOILERS AHEAD YOU'VE BEEN WARNED. Big house. Lots of corpses inside. You have to explore the whole place. As you investigate, the rooms get weirder and weirder, and you find secret passageways to rooms that seem to exist for no purpose at all other than freaking you out. All just level design. Now, there was another cool element on top of that which would be tricky (but far from impossible!) to pace in a sandbox game, and that is that by this point in the game there have been a lot of subtle suggestions that there might be zombies in the city. You haven't seen any, you don't even know if they exist, but as you're exploring this scary ass place with its corpses and bizarro rooms, you start thinking... holy shit are they going to come for me here? Again, completely passive (they don't!). Even the hints are passive, just a couple of suggestive posters here and there and some suggestive dialogue between NPCs. Trickier when you can't be 100% sure players will visit this place or that... but not impossible. Few sandbox games are COMPLETELY sandbox, anyway. There's usually SOME linearity.
I don't think this is impossible. Tougher to make than your average sandbox, sure. But not impossible.
