Well right off the bat I can already see you have one of the pillars of a good horror game on your radar, and that of course is sound design.
Sound is a very import element in a game like this because it controls the mood and gives valuable information to the player about what is happening, so seeing that you already incorporate this into your designing is very good!
Now the following is more generalised advice for any horror game rather than yours specifically :
Lets take a look at a common problem that many indie horror games have and it is probably one of the most important elements to delivering a convincing horror experience, Context.
Why is the player in this situation and why should the player care? There are so many indie horrors that just pop the player in a creepy setting and throw jump scares and spooky black haired girls at the them but fail to explain why the player is in that situation in the first place. You need to get the player attached to the character they control or immersed in the world that surrounds them before you turn the spooky up to 10.
you can have the most scariest events and setting imaginable but without context, without investing the player in the world beforehand, it is missing great potential.
Now having said the above, I imagine you already have some idea for the story of your game and the locations involved, just don’t shrug off the value of building up your setting and story.
You have some good elements that seem to be shaping up to something really cool.
Keep posting updates on your progress!!