What Lovecraftian Monter(s) would you like to see in a future game?

[The Rock Worm]

Another problem is to few of sea monsters. Aside from the Kaernk, Frictional Games doesn't have water monsters.

What about the Rock Worm? ^^ It looks more like a sea creature anyway. And not only that, it can attack from the sea floor, too. And the Shadow is based on Tubifex, which are underwater creatures, so it can be liberately sprinkled everywhere.
With more "normal" enemies, like Deep Ones, I think the biggest problem would be to make them actually "swim" and use the whole 3D space for their navigation, not just the ground.

Both ideas sound pretty good, I especially like the lure-flashlight thing. I found it kinda disappointing that you were so 100% safe in the dark in Amnesia - in Black Plague i shat my pants everytime one of those infected walked past my hiding spot with his light and almost saw me...

Also, since it's so extremely dark in the deep sea you would be entirely blind as soon as you turned off your flashlight - but you have to as soon as you hear an enemy approach, because it makes you stand out like a sore thumb. And if the enemies carry lights themselves, at least you have a chance to escape them. Also bioluminescent organisms would add visual interest to an otherwise pitch black scene. (Not to be overdone of course - we're not doing an avatar remake here )

Another feature that would go very well with the whole deep-sea-darkness-don't use-your-flashlight-thingy would be some sort of sonar. Remember that motion sensor from the human campaign of Alien vs. Predator 2? That thing really added to the horror because it would detect motion around you, while you could neither see nor hear the alien approach.
So you would hear that beep-beep from your motion sensor, but you coudn't tell if the alien was above you or in a vent or maybe right behind you...oftentimes, nothing would happen at all but that damn device made sure to keep you on edge. (Same thing with the radio in Silent Hill)

So imagine, your motion sensor bleeps, you turn off your flashlight and are now completely in the dark. And then it beeps faster as the creature comes closer and all you can do is try and move away from it carefully - you can't really tell from where it's coming or if you're even going the right way, but you don't dare to turn your light back on either... (of course this only works with enemies who don't have a light lure, otherwise it would be pointless)

I wouldn't worry about the regeneration too much, this game could work just as well without combat I think. I mean who would go deep sea diving with weapons anyway? People do it either for science, to search for shipwrecks or maybe to repair stuff like electric lines that run underwater.

I agree that you who goes on a scientific expedition wouldn't carry a weapon. Now while you don't carry weapons in Penumbra: Black Plague, I've seen videos of players who killed an Infected using a box and hitting it repeatedly. Tough Skin/Regeneration would help versus situations like this.

Yes, I have played some of the Alien vs Predator games, so I know how it feels when multiple aliens appear on the radar. You did give me an idea when you mentioned it. As for added scare, What if the Deep Ones were sonar proof? You pick up, let's say, a Dwarf Lantern Shark on sonar when all of a sudden a Deep One lunges for you! Let's say Anglers are completely invisible to sonar, but while the Dunkleosteus ones are as well they can choose to appear as a tiny fish to fool you!

I like the idea of the Rock Worm. I'm surprised I didn't think of it myself. I've heard and seen videos of large deep sea worms that prey on fish (Okay, so these worms aren't monsterous like the Penumbra one and prey on small fish, but that style/template could work for a gigantic Deep Sea Worm).

As for the player himself, we would need something to act as the scuba mask and air tanks. Now finding the wreck of a NAVY Sub, or an unfortunate diver in a Deep One City in the wall of a Deep Sea Trench, could offer 'batteries' for your light and extra air tanks.