--The Middlelands--
Between the Nether and the Overworld lies a dimension that few have gone to: The Middlelands. The Middlelands is a cave/mining dimension with massive oceans of lava, lush and mushroom forests, large swathes of ore, and much more. This new dimension has five different biomes to explore and now works with multiplayer. As I was unable to get portals to work, you must craft a one-time use teleporter to get to the dimension, and a one-time use overworld teleporter to get back.
Recipes:
Overworld Teleporter
Middlelands Teleporter
Biomes:
The Barren Delta
The barren delta is the most hostile biome and is reminiscent of the Nethers basalt delta's. Due to this similarity to the Nether, Nether mobs have inhabited the biome making it the most dangerous biome. Beware of the wither skeletons, numerous magma cubes, and occasional ghast while exploring this biome.
The Dripstone Cavern
The dripstone cavern is a somewhat hospitable biome in the Middlelands, being home to basic Overworld mobs. It is very similar to the Overworld's lush cave biome, just being much more capacious. Water is common, and along with that the glow berries and mushrooms make for a half-decent source of food. Although it's not that dangerous, creepers are very fond of this biome so beware. This biome is great for gathering gunpowder however, required in the TNT recipe and more importantly in the Overworld teleporter recipe.
The Lush Cavern
The lush cavern is the most hospitable biome; the only hostile mob is the occasional slime. Although this biome shows some similarity to the nether (mainly just the warped and crimson fungi that grow sparingly) no nether mobs inhabit this biome. There are small clumps of sand that are spread over the biome, which is essential for crafting TNT, as it is required in both the Overworld and Middlelands teleporter. Overall—just barely being better than the mushroom cavern due to the abundance of wood—this is the best biome to build a base in.
The Mushroom Cavern
The mushroom cavern is the second most hospitable biome, and like the lush cavern, the only hostile mob is the slime. Slimes are slightly more common here than they are in the lush cavern, but still aren't overly common. Although there is a lack of wood, the mushroom cavern makes food seem like a minimally important issue: mushroom stew is the best food source in the Middlelands.
The Dark Cavern