Let's start with that disguise idea. It would allow the player to take on the appearance of said creature, yes? Well, what if it gave the player the abilities of the creature as well. I know that could potentially be a lot of power, so it would be hard to obtain the Enchantment. An Evoker had a 5% chance of carrying one of these enchantment books, though of course this chance can be increased by a looting sword.
However, once you combine the enchantment and the head, whenever you put the head on, you'll become said creature. You'll get all it's health and abilities, though the general rule of thumb is that you can't hold a weapon unless your form can hold a weapon. (Humanoid forms), can't mine and place blocks unless your form can (Enderman), and won't have an accessible inventory (not that you can use it. RAWR XD) unless your form can hold a chest. (Horses, Donkeys, Mules, and Llamas.)
Perhaps heads can be used to keep those type of mobs from spawning if used in some structure?
A simple one would be planning on a beacon or in the way of the beam to make the beacon double as a mob proofing area.
Over powered? Perhaps, but beacons and mob heads are often thought of late game. Plus this would give people what they've wanted for a long time, "A way to build using dark light levels".
If you think it's OP, perhaps just have it adjust the light levels required in the area so it must be 4+ to keep mobs from spawning, instead of the current 8+.(I think it's 8+ now, been awhile since I paid attention to light levels)
This would probably also increase potential dark room mob farm designs as instead of adding reason doors to the ceiling people could just add skeleton, zombie and spider beacons to make an efficient creeper farm. This might seem as easily exploitable, but current mob farms already exploit weird game mechanics.