Nether Abyss [Sub-Dimension]

The Nether Abyss is a new dimension that is set 'below' the Nether. As a dimension, it is unique in the sense that instead of the terrain generating infinitely over the horizon, it generates infinitely downwards (on the Y-axis). The result would be an endless, dark pit, and the only way to get down safely would be to venture down the biomes-ledges that line the walls of the pit. The Abyss would be boxed off by a layer of bedrock on 4 sides - the only exploration option for the player would be to go downwards.

In terms of of what kind of blocks generate here, most of them would be the same as in the regular Nether (so stuff like Netherrack, Blackstone, Soulsand etc.), distributed in layer biomes (same biomes as the normal Nether). There could also be a few new block-types and biomes, that perhaps only generate below a certain depth. One such block could be Crystalized Netherrack, a varient of Netherrack that has arisen from the immense pressure down in the Abyss.

Another unique aspect of this dimension would be the way lighting works: the deeper down the player goes, the less light all light-sources emit (this effect would be subtle and gradual). Perhaps certain light sources would be less affected by this effect, such as Soul Torches. Eg. at a depth of Y=-1000, all light sources would have their brightness reduced by 1%, except for Soul Torches, who's brightness would only be reduced by 0.25%.