Deeper Oceans / Abyss

I feel as if Oceans lack the depth of the rest of the overworld. It would be very interesting to have a special ocean biome that is super deep and at least extends all the way to deepslate if not bedrock. It could be something similar to the mariana trench. 

The biome could be the ocean's equivalent of the deep dark and could be intentionally difficult to see and navigate to without proper gear. This could give an opportunity to include new mobs like giant squids, shrimp, jellyfish, angler fish, and maybe even sea serpents to the game. New biomes and variations of the deep oceans could be included like trenches, underwater geysers, kelp forests, bioluminescent reefs, sea creature bone yards, and ect... 

Perhaps a reward for fighting a boss (at a sea serpents lair?) could be an equipable item like an elytra that allows you to swim much faster and breathe underwater permanently. What if there was a material that allowed you to craft insulated waterproof redstone wiring? Or maybe you could get a special material that allows you to craft a weapon that can summon temporary whirlpools that pull targets down into the water.

This biome could be a way to give more meaning to acquiring prismarine shards and crystals, allowing you to craft glowsticks, a new light source that could float in the water near where you placed it, and could be the primary method of seeing in this new Abyssal Ocean Biome.