The Meaning of Life, the Universe, and Everything.
So I have noticed that while Minecraft has improved in several areas, one thing that Minecraft has lost from its old days was that unnerving small sense of dread that lingers in the background as you play. One very notable example of this is back when Herobrine used to be a huge rumor that would set the feeling that he could show up anytime and anywhere. Of course, now we just laugh at the very possibility of this and meme it to no end. This actually got me thinking about what it would take to make minecraft scary again. Of course, you could just install a bunch of mods to make the game harder and more survival based but that on its own doesn't really make minecraft scary. So I thought about it some more and came to a realization, what could be scarier than being constantly hunted down while being helpless to doing anything to stop it from happening. The idea behind this mod is that these mobs are the predators, you are the prey and you have to do whatever you can to survive your encounters with them or avoid them entirely.
These mobs will have a large enough health pool (say 1000 hearts for example) that attempting to kill them will not be worth the trouble because your goal is not to slay them but run away and avoid them. These mobs will start spawning starting on the first night one at a time no sooner than 5 minutes apart from any encounter but no later than 2 days and can spawn without regard to the lighting level. Both the time they start spawning and the rate at which they spawn should be configurable if possible. You should also be able to turn any of these mobs off if you don't like a specific one for some reason.
The Cave Beast:
The Cave Beast will spawn as the name suggests, in caves about close enough to be heard but not close enough to be seen. It has spent so long in the caves that it has lost its humanity and will aimlessly search the cave for either diamonds or miners as both act as food sources for it. It has the appearance of a monster with a huge, bulky body and ginormous mouth large enough to consume any human. Despite its large appearance, it is rather fast (about as fast as a spider) and has enough strength to burst through walls. A hit for this things pickaxe is strong enough to penetrate even the most refined of armor. If it sees diamond items or ore, it will take a moment to stop and eat it before resuming what it was doing before. If you hear large foot steps and mining that isn't coming from you, The Cave Beast is likely close by and can hear you if you mine or walk nearby it but not crouching. If you hear this things scream, RUN.
The Hunter will spawn on the surface. They are a trained sniper who is very effective at taking out his targets but unfortunately, you are that target. They tend to spawn a couple hundred blocks away and blends in with his current terrain. The sniper will take a bit to find you at first but once they do, they will aim their shot and fire. If you see a red laser pointed at you for a second and disappear, they have taken the shot and you need to hide behind some non-glass cover. (No, shields won't help) They will then take time to reload their shots for 5-10 seconds before taking another shot. If you get hit by one of their shots, you will die INSTANTLY no matter how good of armor you are wearing. If they lose sight of you, they will send their hunting dogs to sniff you out and try to kill you. The hunting dogs are about as strong as a zombie and can be killed. If you somehow manage to get close enough and try to hit them, they will simply use their ender tech to teleport away and reposition themselves to take the shot.
This is all the ideas that I have for this mod that I came up with. Now I think there is room for a lot of other ideas like an ocean hunter or nether/end variants of these mobs but I think that even just this would do a lot to make Minecraft a lot more tense. Feel free to suggest improvements to this idea and I hope this isn't too complex of an idea to attempt.