The dark biome

 The Dark Biome is what I'm thinking, it could be a deep and damp biome infested by tall deformed trees which cover the sunlight with their grayish leaves and dark black wood, in the biome, there could be implemented a new type of seed/crop which would give you a great amount of food but also a negative effect such as slowness or blindness, it all depends on your luck whether it will be something harmfull or just a mild inconvenience like slowness. Some of the new mobs can be like the other biome-specific enemies but this time instead of a skeleton or zombie how about an enderman being  biome specific, my idea is that it would have the same concept of a normal enderman but instead of the enderman just attacking when seen how about it inflicts a short blindness effect, these enemies would be rare but a nice surprise since they could drop some sort of new item needed to craft a more useful item, I have a few ideas as to what that item would be , maybe the item this enderman variant drops would be something like a corrupted ender pearl that has a similar effect to a normal one but with a slight twist. That said, In the biome there could be gravestones that when broken would summon zombies or skeletons but apart from that something good, like damaged armor or weaponry, not to mention that it would allow brave players looking for a fight with worth-while gear.