Void

Imagine 5 levels of reality (minecraft reality anyways).

The Overworld in the middle, the Nether below the Overworld, The Aether above the Overworld and the End at the top, but what's at the bottom?

Answer: The Void!

This Dimension is always dark and torches are extinguished when placed and all other light is eaten away. You can get to this dimension either by falling into the void or after defeating the Ender Dragon the portal might malfunction and instead of sending you home it would send you to the void. Oh, you can also get to the Void by sleeping and some nights you get a Nightmare of the Void, but its not just a dream. Shape shifting mobs live in the void called Fakes and chests that look like they got good loot in them might be terrifying monsters called Things. There are also villages filled with Ghosts, Wraiths and The Forgotten. What are the Forgotten, well riddle me this. Why aren't there so many regular Zombies in the Nether? When a Person looses his mind, he becomes a Zombie. When a Person looses his flesh, he becomes a Skeleton. But, when a Person looses his soul the soul becomes a Ghost and the now soulless body becomes a Forgotten now forever wondering the void to take a soul to make him whole again.

In short the Void is super scary and the only way to get out the the Void is to find a Spirit, a mob that spawns only once in the Void. It can't be killed and if you right-click it it would return you home and reward you with a Aura Orb which you can either smash it to give you 100 experience or you can craft it with an item to increase the levels of all enchants on it by 2.

I think there should be more ways to get out, but I do like the overall idea. I imagined this dimension as a mashup of Nether and End: giant caves and floating islands alternating each other above the endless abyss, everything made out of bedrock with some patches of a harder-than-obsidian-to-mine weak,cracked variant of bedrock, maybe hiding treasure

Hmmm... maybe horizontal Nether Portals high up on the "roof" of the Void since it should be below the Nether, or,what about randomly spawning strong light sources that look like beams of sunlight.The beams should inflict you with Levitation as your first person view becomes brighter and brighter 'till it's completely white and, a few instants later, you're back in the Overworld

You could bring obsidian with you to build a portal so you can get to the nether, and the beams of light sounds awesome as well! Just the only way to get a Aura Orb would be to find this special mob or do the light thing you're talking about. This would mean that these beams of light would be rare but not as rare as the special mob. Plus, maybe there should be some guardian defending the beam to balance it out.

Maybe the beams could spawn ONLY inside structures, protected by Ethereal or Lovecraftian looking beasts, that may drop an Item needed to activate the beam by using it on a block in the middle of said structure.Also, if you take one of these blocks and bring it trough another beam, it should create a black vortex when activated, inflicting you with slowness, blindness and nausea before teleporting you into the Void. Those Vortex/Beam activator blocks could be craftable with a very expensive recipe, maybe one needing one of your Aura Orbs, thus forcing an unwanted(probably) trip to the Void and consequent searching and finding of said special mob that gives it to you.