You missed a couple things from your original compelling features list:
Story - You're not going to manage to make a good story. From what I can see your not a literary master.
Challenge - Think Demon Souls, this is something you could actually implement relatively easily, just make sure dodging attacks works properly.
To really make a horror game you should reduce the draw distance in certain areas, as it stands you're gonna see what ever threatens you well before it attacks which the horror element. All horror games and movies rely on the imagination, you have to make the audience feel dread and anticipation. You'll need to put it in first person too, you can't really scare someone when they can see what's coming up behind them.
Oh and is it just me or are you trying to make a console game for mobile? Perhaps you haven't realised but the top mobile games are arcade style games not long RPGs.
Yea I was hoping to find a writer at some point but who knows. On other forums people were telling me not to worry about it and to just go more for a hacknslash Diablo esque dungeon crawler and I kinda agree.
I like the idea of demon souls, I was thinking of doing the particle effects from the slain monster to the player as collecting "souls". Sounds a lot better than "monsters killed".
What few sounds I have added help a lot, when doors open/close they make a sliding sound, chests make a squeaking sound as they open when you enter their collider. When you enter a mob spawn it makes a scary screaming sound. I need distant screams and scary noises to add throughout, putting triggers in hallways to trigger distant sounds should increase tension and make people wonder which way to go next. Also playing a monster coming down the hallway growing closer and closer then having the monster drawn when it's really close would do a lot for buildup.
I put my monsters on a separate layer and can easily adjust the distance they're drawn. The cavern and dungeon sections are way more efficient than the levels with terrain. If any of the terrain is in view it draws the entire terrain, and it's way too many draw calls for the phone. In the cavern/dungeon segments I cull whatever's outside the walls and it gives great performance (35 draw calls, 26k poly). It only draws the next couple sections in view and drops anything outside the walls even if it's in the camera frustum.
For monsters I can set a trigger and however many spawn points tied to it placed wherever. When you enter the trigger it plays a screaming sound and spawns monsters at it's spawn points. If you leave the area without killing the monsters, it starts a cleanup function that delays and destroys the spawned monsters. This has helped a lot.
But yea it probably is too much like a console game. Do you think hacknslash diablo is too much for phone? This runs fine on kindle fire thus far.