I know this has been suggested a few times by other people, but another take on it won’t hurt anybody
As has been pointed out before, the dimensions in Minecraft sort of correspond to the classical elements. The Overworld is earth, the Nether is fire, the End is air, and water is missing. So it deserves its own dimension: the Abyss.
Now in other people’s ideas for a third otherworld I’ve seen people claim that you’d need resources from both the Nether and the End, but I don’t like that. The “End” needs to remain the “end” of the game. If anything the Abyss should contribute to getting to the End. But anyway.
The Abyss is accessed by portal like the other dimensions. Its portal are like a mix of Nether and End portals. Like the Nether portal, they are composed of blocks you can obtain and position how you like. Like the End portal, the they are horizontal and entering it is like jumping into a pool. The corresponding portal floats in the Abyss, and can we swum back into at any time. Spawning back into the overworld finds you standing in the portal, which won’t teleport you until you jump out and back in.
The portal needs to be made of Prismarine Crystal blocks, made by 9x9 crafting of Prismarine Crystals. These are arranged in the same way as an end portal (or larger if that’s what you want). To activate it you need to place a water block against the inner rim of it, like setting fire to a Nether portal.
The Abyss needs to be very distinct from the Overworld Ocean. The same way that the Nether is very different from a cave with lava in it and the End is very different from the occasional floating islands you find.
First things first: the Abyss is totally filled with water. It is possible to create air pockets, and a few will naturally generate the way lava lakes generate in the Overworld caves. Bubble columns from the depths should also help the player stay alive. Being underwater tends to be pretty difficult with the light levels. The water getting darker as you go deeper is a more gradual in the Abyss. Up in the top layers, it should be nearly as easy to navigate as in air. In the bottom layers, it’ll be nearly pitch black (natural light sources not withstanding) and require serious stocking up on night-vision potions and sea lanterns.
Like the Nether, both the top and bottom of the Abyss are covered with a layer of bedrock, though the Abyss shouldn’t feel like a cave. The top layer of the Abyss is a layer of ice (and packed ice and blue ice), as if the surface of the ocean is frozen over. Giant ice stalactites spear down into the water. Air pockets and other goodies are waiting for you inside the stalactites. The ice doesn’t freeze the water around the stalactites or regenerate them, though the very top layers until the height limit is reached contain an increasingly rapid water freezing effect that ensure the layer of bedrock is impossible to reach (maybe there shouldn’t even be one? Just an impassable layer of ice? Or a new form of bedrock ice?).
I haven’t made up my mind about what belongs in the middle layers, other than otherwordly fish. Open water seems just boring, but is probably more practical. The floor of the Abyss depends of different biomes (to match the Nether’s upcoming biomes) which I’ll expand on later.
The bottom layers of the Abyss before the bottom bedrock are made of “Abyssalstone” (equivalent of Netherrack or Endstone, deep indigo in colour). There are a few caves in there, some flooded and some containing air pockets. There are also ores to dig, rewarding you with Prismarine shards, Prismarine crystals, and maybe something more substantial that couldn’t already be harvested from Ocean Monuments. Atop the abyssalstone is a layer of sand and gravel, or maybe a unique block with similar properties. Also some magma blocks to provide air columns.
The Abyss needs to be an equivalent realm to the Nether, and since it’s receiving new biomes the Abyss should have them too. I’ve heard they’re working on 3D biomes but I’m not smart enough to try and implement this in my proposal (maybe the layer of ice could have its own variants?). These are a few examples, and should be the absolute minimum of the biomes included:
Coral: Abyssal corals come in far more variety than the Overworld corals, covering every colour not covered by Overworld coral. The Coral block structures are also much larger than Overworld coral reefs, forming enormous tendril-like towers to match the ice-stalactites. Airpockets and other goodies might be found inside the coral towers. The design of each tower depends on the type of coral. Some of the coral might be luminescent to help the players see.
Kelp Jungles: Very different from the kelp and seagrass of the Overworld. Here it grows tall and thick, and resembles the trees of the Overworld jungles. The player should swim over them unless they want to get lost. From these kelp-trees the player can get a unique green-coloured wood, and the decaying leaves will drop otherworldly fruits. The leaves and branches might hold air pockets. Some trees grow glowing fruit that help the player see.
Ice columns: Very simple. Where the ice stalactites on the roof are much larger and extend all the way to the sea floor. There are no lights here, or magma blocks to provide air. A very inhospitable biome unless you’re lucky enough to burrow into an air pocket.
Plains: No fancy terrain features, except maybe hills and trenches that occurs in all of them. Just the bare floor of the abyss. Useful for generated structures.
Other ideas: Brine swamps? Magma fissures?
In other suggestions for an Ocean Dimension I’ve seen mob suggestions like “whales, sharks, rays” and all that, and I have to disagree. All those are natural species and belong in the Overworld oceans. The Abyss is an alien world and should be at least as alien as the Nether mobs. Here are a few suggestions:
Merfolk: A humanoid mob with fish tails. Merfolk are the Villagers and Piglins of the Abyss. Friendlier than Piglins but more distrusting than Villagers. Like the Villagers they get different variations depending on where their village spawns. They can defend themselves like Piglins, though also have dedicated guards like the Villagers. Which brings us to:
Abyssal Guardians: A variation of the Guardians from the Ocean Monument. Neutral rather than hostile, and focuses primarily on physical attacks than putting status effects on their foes. The player is capable of crafting this kind of Guardian to defend their own ocean bases.
Zombie Merfolk: Self explanatory. Tend to keep to the undersea caves or the ice layer.
Skeletal Dolphin: Kind of a silly idea I'm only partially sold on.
Sludge: The Abyssal equivalent of Slimes and Magma Cubes, coloured like gravel or sand or something. Smallest are harmless, like Overworld Slimes.
Kraken: Monster A from Minecon 2017. Very dangerous and lurk around the generated structures. Maybe hint they're magical or mechanical?
Murkmite: A mix between bees and silverfish that have nest blocks on the sea floor. Hitting one causes large swarms to come after you. Are a prime source of food if you’re up to fighting them.
Leviathan: The first thing you’ll learn to hate, and patrol the open water everywhere its big enough to spawn them. Resemble giant crocodiles with flippers, but each flipper is another mouth.
Mobs currently in Minecraft: Guardians, Drowned, I guess fish can inhabit the sea, and Endermen can rarely spawn in air pockets.
Just like everywhere else, there needs to be structures to encourage exploration. I don't have many ideas here, sorry.
Merfolk Villages: Found in different styles, some built into giant coral or kelp or ice columns, or some just by themselves on the oceanic plains. Constructed mostly with the kelp-wood, but also have some Prismarine so we know who build those Ocean Monuments Or maybe not, maybe we should leave that ambiguous.
Abyssal Citadels: Equivalent of the Nether Forts. Resemble Ocean Monuments but should put them to shame. Inhabited by the undead, Krakens, and Guardians. Should have some major rewards. Like, rework the progress to the End so its necessary to visit these structures type rewards.
Submarines: I'm a little unconvinced on this one, but I'll keep it for completion's sake. Similar to End Ships, sort of. Rare vessels in the open water between the ice layer and the ocean floor. Designed sort of like real pre-industrial submarines, and contain air pockets. Built long ago, so are slowly beginning to decompose; they should be partially flooded but still contain goodies.
I really really like all this. Water being my favorite biome, I’d love to see it get an extreme expansion in this way. I do think that, like the end and the nether, the abyss should have a new boss associated with it. Not sure if it should be a naturally spawned boss like the Ender dragon—I would think it should be player-spawned, like the Wither. Perhaps they should be spawned with drops from Krakens or Leviathans? Or else, maybe we need a new boss spawning mechanic. Perhaps every Abyss could contain exactly one of some kind of fortress (like the end fortress in the overworld) that has to be located, and within it is some really substantial treasure (not like in ocean monuments), but guarded by this new boss. Not sure what the boss would be though.
I like it, but other than the abyss, I think “The Deep” would be a more fitting name, since the abyss is usually interpreted as a infinite black void after death, or it is also referred to as a fantasy take on hell, also I just think The deep sounds better.
I think instead of having to create a portal, their should instead be a small chance to come a cross an area in the ocean ravines called a rift that would act as a portal between the two planes of existence.
I think if you kill the sludge it should drop a ball of poisonous foo and on death it creates a poisonous cloud like squids or lingering potions, they should be really cute as well to add karma to killing the cute creatures.