I have been putting quite a bit of thought into this, and would do it myself if I knew how to code or program it. Anyway, we have the Nether, a dark, volcanic underworld flowing with magma, and the aether mod, which is a heavenly sky world with floating islands and a very light feel to it. Well, what if there was a mod that allowed you to turn the overworld, the very soil you start the game on, into a dark, abandoned, ghostly area?
It would have the same terrain generation features as the overworld, but replacing most blocks with a more demonic texture during the time you are in the dark area. The sky is a light brown, grass and trees are dead, mobs are skeletons or spirits, and abandoned towns can be found sometimes. Water is replaced by a green acidic material, and volcanoes can be found periodically. It would have its own item tiers, making overworld items ineffective, with the same strength as wood tools. (much like the aether mod) darkwood, bloodstone, steel, and Light. Light ore generates at diamond level very rarely, and is the most effective tool against enemies and blocks in the world. The portal to the Dark World would be crafted out of slowstone or netherrack and activated by lava.
I know this would be a pain in the ass and take months to code, but in my opinion it would be well worth the time to have something of the sort.