I know that there are a lot of suggestions out there, but I have a suggestion that could be added into a later minecraft version, several suggestions, actually.
My first idea is to incorporate a new dimension; I have decided to call it the dark dimension because I do not have a name for it yet.
First for the portal, it would be constructed by collecting 10 dark crystals, these would be available as a mob drop for (possibly?) phantoms. With nine of these you would craft the frame of darkness. The frame would be textured similarly to that of an ender portal's frame. The frame of darkness will reduce light levels in the surrounding area by 5. When twelve of these frames are aligned like an end portal, a dark portal will appear between each frame block. Entering this portal will kill the player, unless they are holding a dark crystal, the tenth one collected earlier.
Secondly the layout of the dimension. The dark dimension would have a light level of just 1, and all light sources will attract shade skeletons, purely black skeletons that wield shields and maces, the new weapon to be introduced. A mace will function as a high damage weapon, dealing the same damage as a diamond sword, and having the cool-down time of an axe. The mace can be enchanted with the Sharpness enchantment only, but will not be naturally enchanted at all. It will have 85 durability points and will be crafted by placing an iron block in the centre of the crafting menu with a stick underneath. Three iron ingots will be placed on each remaining adjacent side of the iron block. Shade skeletons will have 24 health points.
The main block of the dimension will be shadestone, textured like cobblestone, but using black and dark grey. This will make up majority of the surface, the rest covered in either ruby ore, which will be textured like nether quartz ore, but inside shadestone and a ruby red for the actual ore, or obsidian. Rubies will not be used to craft weapons, but can be used in trading with villagers or increasing the power of enchantments in the enchantment table. Below the surface, all three blocks are also found.
The generated structured in this dimension will be dark castles. Generating the same way as nether fortresses, but made entirely from obsidian, with shadestone slabs, fences and stairs, perhaps with a different layout to nether fortresses to not be repetitive. Mobs found throughout the dimension will include Witches, Shade Skeletons and Phantoms.
Alternately, if this is too much to implement, how 'bout just Snakes. Providing the player with snakeskin as a potion ingredient, which can be found like eggs, when a snake sheds and venom as another potion ingredient, as well as venom being drinkable at a price - It will give you nausea and poison 1 for eight seconds, but if survived, will repel hostile mobs for three minutes.
Please note that soon I will add images of textures, etc. into this thread, and refining my ideas.