Get ready to read, dude. I just went on a field day coming up with ideas.
1. I would love for it to be able to switch between the two,but if you can't,I'd stick with first person,as it make's scares way easier.
2.If you want to stick with such a dark area, I'd look at prisons, fortresses, space ships, and such. Hell it doesn't even have to be that exotic it could be something as simple as an Apartment. Every room has it's own distinct style, some rooms are left open for optional exploring. A single apartment.
3.I WANT to know the main character. I want to know their backstory. I really want to have a good reason as to why my character is doing what he is doing. With people like Isaac Clarke I understand why he doesn't just put a gun to his head and end it all, but when I see people surviving these situations with no knowledge of their past, I always think "No one would live through this,most people would have just given up by now"
4.I want weapons,but I don't want them to always kill. I would like it if they just stunned, and got up after a certain amount of time,depending upon the players health and stamina and such.Once they got up,their walk speed and damage was reduced. I want the effect to be shorter the lower the players health is,so if the player has a full health bar and is in tip-top shape, hitting should either kill them or knock them out for an extremely long time. If their health is halfway, the majority of enemies should be knocked out only and should rise in 10 or so minutes with half their original health and speed and power. If your at low health and your hero is limping while walking and breathing heavily, I want enemies to get knocked out for a short time and when they wake up,all their stats are restored.
5.Alone and if Im with an AI controlled group I want them to be ESSTIENTAL to the story. Even if they are a just for a plot twist, like near the end or beginning,you find out your allies have been your partner just talking to himself like Mason in Call of Duty:Black ops.
6. If you do multiplayer,don't be afraid to do some cool modes with it. Do like a SAW style game where you must save your partners from death by solving puzzles with other friends,or half one player set up the enemies and jump scares. If you just wanna do strictly co-op, make sure to separate the group ALOT. No one is scared when they are in a group. Imagine playing something like Dead space 3 or Left 4 dead, and all of a sudden a cutscene or whatever happens and everyone is separated. It would be scary, and it makes you feel as though any second you could be all alone once again. When playing a MOBA game,traveling in a group of players gives you this sense of security and power, and running from a group of champions in a moba game gives you a sense of fear,knowing that if you slow down or they get within range for even a split second, you will DIE. I want that in my co-op horror. When in a group I want to feel like a badass but I want to be reminded that alone I am the weakling. I want to be reminded that our survival depends on our teamwork.
7. If I were you I would just try to do it Dead Island,or tomb raider style. It's a free roam game,but the missions themselves and certain areas are scripted.
and if you choose one of these, you DAMN WELL BETTER MENTION ME IN YOUR GAME! Just kidding, a free copy will be just fine. :)