Since we got like 2 new world mods, (Aether and ice world) i think there should be a dark version of your world like in MP 2 echoes, Dark Aether. That would be awesome and there would be some items you can only get there such as a armor that slows the damage rate of the toxic atmosphere. The portal needed to get there has to be made the same as Nether portal but with iron blocks, That would be a challenge to make the portal because you need so much iron. It can only be opened by luring a creeper into it and making it explode inside it.