Here is my completely far and not overpowered idea of a new mob: The Griefer!
He weres full dimond armour and has a diamond sword. He can see though walls and can destroy blocks to get to you. He can also build upwards to get to you too. Griefers are attracted to player-placed objects and will randomly destroy blocks. He can also place TNT randomly. They are extremely attracted to chests. if you manage to kill hime he drops his armour, sword, and 1-10 diamond blocks, but when he dies,a lava sorce spawns wich has a very high chance of destroying stuff
Requesting this thread to be deleted because this wa joke, not a real suggestion.