I believe that The End needs more mobs.
There are just 2 mobs that naturally spawn in The End and i believe that the addition of more "Scary" mobs that look more "surreal" or borderline "nightmare-ish" (like The Phantom) would be beneficial to The End to add more variety and challenge.
They don't HAVE to be aggressive,just neutral,for the most part at least.
They can vary in size and shape (depending on the mob) and should drop rare items (to make the jurney wotrh the risk),but they must be hard to defeat since The End is a dimention that is usually explored during the "late game" (after defeating the Ender Dragon/Whiter),and the player (usually) has good weapons and armor at that point.
I have no exact idea how they should look or sound but i think that they sould make the player just a little bit nervous to get closer the first time they see those "things".