Conscious mobs.

Make all the mobs and monsters as smart as Iron Golem. All the players bully him and beat him with just three pokés (I mean, they rise up), but this time make Iron Golem smart and keep his distance until they lose patience, then betray them in the end! This would be suitable for hardcore mode Part Two: The Cunning Creeper! When a player is hiding in their house, they'll deliberately blow it up to make room for monsters to kill them! Or when a Creeper sees a Minecraft player underground in a cave, they'll charge themselves up, then fall and explode! Improve it a bit and increase the difficulty. Part Three: The Zombie Digger (meaning it breaks blocks). This zombie will break any block in its path—wood, stone, anything—and they're stronger, carrying swords and armor. Even small zombies should be made very difficult to handle. They can move as fast as a normal player and move randomly and unpredictably. The final part: The spider. Make the spider move a little across the wall, I mean, move its leg when climbing. You've made it crippled, stuck to the wall. It's unnatural, a farce. And the spider releases a sheath to calm the player.