All right, the game seems to have a lot of interest already, which is always nice. The general consensus seems to be on "difficult and realistic.". What degree of realism is desired? Are we talking Phoenix Command level "Every Bullet is Tracked" style battle simulator? Or a more abstract version, like Gurps?
Also important is the food and resource use. Realistic and difficult means using these realistically. Should I add them? An idea I had for this is for an in-game clock to countdown the time, making it simple to tell when you need to do something, rather than a more narrative time. Ex:
10:00. You get up and start searching for food.
11:00 You stop, getting hungry.
Except, of course, a lot more detailed. Using these and figures from Google, I could probably make a somewhat realistic basis for how much food and water you need over time.
As for the realism of the enemies, how should I handle that? I am, of course, not going to let you on to everything about them. Mystery is what makes them what they are. But should the cultists have access to actual magic? Technology that looks like magic? Should the Mythos be simply poorly understood creatures of flesh and blood, or fully beyond human understanding? Is the sanity loss caused by magic, or just your human mind cracking under the influence?
Also, should I add a SAN meter like COC? If so, how difficult should it be to raise? I would imagine that living in the world as it is would make you a bit harder to drive insane, though a lower number would make the game more difficult. Should I keep the SAN meter hidden, so not even you know your own sanity?