NetherEx is a large reworking of the Nether, focused on three core ideas: Expansion, Exploration, and Experience.
The mod currently adds four new Nether biomes with each designed to fill a specific purpose within the Nether. Each biome is different and has a unique approach compared to the others. The biomes are populated with new mobs, decorated with new blocks, and filled with over a dozen new items.
NetherEx 2.0.0+ requires a library mod, more info can be found here.