The Nightmare. It's a giant vampire bat that has red eyes. It spawns in a new dimension, The Death Caverns, a huge cave, but that's all the dimension is, the cave is about the size of the end. To get to this new dimension, you have to go to the nether. In nether fortresses you can find an unlit nether portal, but it's made out of bedrock instead of obsidian. To light this portal, you need a nether star. Once you enter this realm, there are tons of ores to collect, but before you can get rich with all these ores, you have to kill the boss. It's like the ender dragon, it breaks blocks except for bedrock. This new boss also doesn't destroy the portal block and doesn't destroy ores, but it does destroy stone. Upon death, The Nightmare drops a sword called Fang Blade. This sword gives the wither effect and has the strength of a diamond sword, but infinite uses. It can only be enchanted with an anvil. It's very useful for fighting the Ender Dragon.

i think it should be a Demon that spawns in your house when you sleep and burns your house down

it has 100x the amount of player health

killing it would get you a demon tooth to craft a demon stature

I think there should be a boss called the Diamond Golem. It would be like the iron golem, but its evil and made out of diamonds

It would have 10X the max health of a regular player

You can only spawn it in the end after you killed the enderdragon

Then it would give you a special crystal that increases your max life by one health

I believe a thing called a nether shrine should spawn in the nether randomly, Being like a stronghold. Once you have put 5 of each skull available in a big x pattern you must sacrifice a wolf to it. And a giant Mother blooping demon erupts out of the ground carrying a giant sword which must be avoided or instant death. It calls upon things called Bloodletters (Which would be semi-powerful demons that fly carrying battle axes, Which after death there weapons can be picked up and used. THey also drop one demon heart.) Once killed it literally explodes into thousands of particles. Where it dies it drops two things. Demon metal and a greater demon heart.

You can make armour from the demon metal which is slightly better then diamond. When a demon heart is added to the middle of the chest plate it increases speed and overall everything when wearing. Using a greater demon heart quadruples all effects used by the regular demon heart. But you can only get one greater demon heart in the game. You can also put the the demon heart into a beacon greatly greatly increasing its range and strength. (If you were to put the greater demon heart in the beacon Id suggest making an emerald pyramid to the sky barrier.

You should be able to find a Big Mushroom Lord in Big Mushroom Biomes that looks like a big Robot (1 Head, 4 arms, Hover pad for feet) that cannot move and shoots Monster spawn eggs for a Mushroom soldier. The Mushroom Lord is also Surrounded by 4 Giant (Bigger than normal Giant) Red Mushrooms that Protect him and Shoot poison potions at you. The Soldiers have low damage and low health but the Mushroom lord spawns lots. To kill the lord you have to chop down (axe) the big mushrooms (Takes as long as Stone with an axe) Taking down the Highest block on the mushroom deals 1/6 damage. Once all the mushrooms have been destroyed you deal the rest of the damage with normal combat. When he dies he drops 2 Special blocks called Brewshroom Blocks, 1 red, 1 brown. You place it and you can get infinite Mushrooms from it (Right click for a stack). He also drops a Mushroom Essence (Item) and you can craft it with a Chest to make a Special chest that if you put any sword in it it enchants it with poison, the enchantment can't be got with normal enchanting table. The mushroom soldiers sometimes drop Red mushrooms and Rare drop of Mushroom stew. Also add a new mob, MooshSpiders that are like normal spiders and can be jockied by Mushroom soldiers (Bonus melee damage, no ranged). The spider sometimes drops fermented spider eyes.