Wow I'm really enjoying these ideas, so I'll just go on ahead and toss some stuff in.
3D or 2D: 3D would be much more frightening, 2D would be more stylish, but nowhere near as frightening.
There should be no heads up display of any kind, like Call of Cthulhu: Dark Corners of the Earth
Dreams dreams dreams, in Lovecraft's works, dreams are very important, and they should be in this game. Perhaps the reason the character first enters the mention is because they are plagued by terrible nightmares of the manor house and the horrible things that happened within? The dreams could also be used to keep the player in the mansion, for example, if they flee outside (The first time.) After they get so far The PC wakes up on the porch of the mansion, or on the floor in the "safe room".
Handling save games without derailing the atmosphere; Obviously, people need to be able to save their game so they can work, eat etc. or just so they can go back if they die or whatever, while it's more atmospheric to have the game end entirely when you die, this can be very frustrating, particularly in a game that doesn't see fit to give you tutorials. This can cause a lot of players to give up in frustration, and considering you probably want people to play the game.... well. I was thinking perhaps the player character writes in a journal before they go to sleep? This could also be used to frighten the player or give them clues, if they don't know what is written, but can look at it later.
The nasties! It'd be awesome if at the beginning, the nasties can't be seen in the light at all, and theres nothing there, not even a squeak or a puff of smoke to indicate anything was there, this would go on for long enough for the player to get used to it, THEN the nasties would start to be a little more visible.
Wow, long post, but if this game ever goes into development, I want in! The idea is fantastic! I'm somewhat qualified too.
Edit: I know CoC was a mostly crappy game but it had some good stuff.