I would suggest adding some sort of lightning system, not only for stealth but to spice up the combat.
In most roguelike games ranged combat is kind of dull, without being able to take cover or aim manually it boils down to be able to shoot down enemies when they are within your view radius or not being able to when they are not while at the same time trying to minimize the amount of them that can shoot at you. DoomRL probably has some of the best ranged combat in RLs in general, but maybe it could still be spiced up a little
I think adding a lightning system to the game that influences gunplay and stealth would be fun. Most areas should have the default ambient lightning, making it play similar to regular DoomRL. But other areas (which I will call dark levels) could have no ambient lightning by themselves but instead sprinkled destroyable light sources to change the pace.
This should be the levels of lightning, for example, a point light source like a lantern should create varying levels of lightning around it depending on distance. Shooting a gun also creates a point source of lighting for an instant, letting you take a peek at the area. But of course, there are better ways to go about it than making light and noise to attract monsters.
Level 0: Pitch black.
Only enemies adjacent to the player can be detected and only adjacent tiles are mapped. Great for stealth against enemies whose main way of detecting you is sight, like former humans and machines that operate at the visible spectrum, like security cameras. Not as good against monsters.
Enemies that shoot in your direction take a huge penalty to accuracy if they shoot you while you are in the dark. Same goes for you. Whoever it also has disadvantages, reloading and usage of inventory items takes longer unless you are using some item or trait that improves your visibility.
This is the kind of lightning present in areas with no sort of lightning whatsoever, like completely enclosed rooms. This areas are rare, most of dark levels will use level 1 lightning instead when there are no sources of light nearby.
Level 1: Very dark.
You can see and shoot enemies up to three-four tiles away with no penalty. Further away than that you start having trouble hitting them and even detecting them. Further than 4 tiles you can detect their presence but you can't see what kind of enemy it is. Further than that you can't see them at all. Accuracy penalties are not as steep, but they are still there. No penalty to speed while managing inventory or such.
The mapping of the area is only reduced by two tiles. For the most part, dark levels use this lightning, switching to level 2 as you approach lightning sources.
Level 2: Dark
The default lightning level for most of the game, which is to say, the ambient lightning in regular non-dark levels. Works like DoomRL did or whatever is the default in Jupiter Hell.
In dark levels it acts like a transition between Levels 1 and 3: Enemies in level 2 lightning areas will be detected, recognized and shot as if they were in the above (default lightning level). Same goes for the player. This is the kind of lighting present when there a light source illuminating the area directly, but somewhat far from where you currently are.
Level 3: Light
This is the lightning level when close to a light source. When standing in it inventory management and reload speed, accuracy (even against enemies in the dark, though not enough offset the defense the dark provides) is increased. But at the same time, accuracy towards you while standing in it is also increased and the radius at which someone standing in a light area can be targeted is 1.25 times the regular one.
Level 3: Light Source
Same as above, but with a catch. Light sources only occupy one tile, but their location is always known as long as you have direct line of sight to them (it shows up on the map or maybe as a cursor), same for monsters.
Light sources can be objects (maybe barrels on fire that can be pushed or shot to destroy them, also, lamps), maybe a special type of monster or the player's flashlight.
There could be interesting mechanics with light, for example, against armed former humans hiding in the dark could certainly help against them. But of course, they should be smart enough to know this and avoid it. However the player could force them into the light by using flares (maybe throwable items?), switches to turn on the light in their area while basking in the dark himself.
Against monsters and demons, because of their nature I don't think darkness would be a huge problem to them, so the player should do the opposite, try to light everything as much as possible for the increased range and extra benefits. Light maybe should also hurt or make the monsters less of a threat in someway, while staying in the dark exposes you to being mauled.
So in a nutshell, former humans are better dealt with from the dark, monsters are better dealt with from the light. This forces the players to decide which considers a bigger threat and act accordingly.
There should also be items meant (and maybe selectable traits) for the different situations. Night vision googles are meant for exploring the darkness, but can break easily during combat and their electromagnetic activity can make defense sentries and robots detect you. The flashlight lights up the area but exposes you to former humans and things with good eyesight. Traversing the darkness blind exposes you to monsters. Night vision scope increases your accuracy in the dark but does not actually help at navigating dark areas.
Maybe there should be more to the mechanics, maybe they are too complicated, but overall, I think maybe having some of these system might make things more fun (though slower paced) to some degree, what do you think?
