Making survival horror elements?

[Hidden Hands]

Got a few problems here. I need my game to outline a few survival horror elements. But I'm not entirely sure how to pull this off. I need to achieve the following:

1. A trigger to make an enemy ahead of you dissapear behind a wall as you approach it only for it to be gone, and not there. (The hallucination and/or paranormal effect of a monster you cannot truly catch up with.

2. Triggers that start a mini in-game cut scene revealing a monster for the first time.

3. An indestructable monster that kills you to trigger the start of the next level. This monster needs to be unkillable and also needs to kill the player to transition him to the next map, however, without the "kills, secrets, items" screen in between so it appears seamless.

4. Limited weapon capacity. I want my game to allow you to carry only two weapons at once. A melee, and a gun. If you have say an axe, but want to pick up a crowbar etc. then you have to drop the axe first and take the crowbar instead. BUT it needs to remain on the floor so you can re-switch back to the axe again if you want too later. Same with guns etc.

5. Foggyness and draw distance. I want the game to have a low draw distance and cover much of it in fog, sort of similar to Silent Hill.

6. Compound mixing. To make your own ammo and health from raw materials you find.

7. In game triggers to switch maps to create the illusion of being in a darker, parallel universe in the style of Silent Hill.

Any help will be appreciated, thank you in advance.

[Voros]

Reference mods:
http://www.mediafire.com/file/1jcrkt8zllq5anz/
http://www.mediafire.com/file/atqjba864y7qqaw/

Also have you considered using silent teleports? They can create some weird scenarios (I'm pretty Going Down did this several times).

[R4L]

The 1st one and 2nd one can be easily done. The 1st one, you would just move a monster when the player walks over a linedef, and silent teleport after.

The second one, use cameras. Disable player movement, and you can use an aiming camera if you want a certain angle, or a moving camera if you want a more cinematic approach. Again, activate via linedef.

Three, I'm not quite sure. I think you'd want to use a hub map system for that. I know Unloved doesn't have level end screens when you visit other levels.

Four, again easily done. Two custom weapon slots I'd imagine, with their own ammo types. Make the player start with one of them.

Five, I believe GZDoom does fog, but it can be very laggy. I know on The City of The Damned 2 my FPS is choppy when there is fog, and I'm running rather decent machine... The only reason SH had fog was because of the PS1's limitation, but I'm sure there is a way to simulate that so it wouldn't bog down FPS.

The last two... Mixing ammo types like RE3 shouldn't be too hard, but changing all the textures to be like the Otherworld might require some work, and even then idk if you would be able to fade in new textures like SH: Homecoming did.