The Meaning of Life, the Universe, and Everything.
I was reading "Arthur Spiderwick's Field Guide to the Fantastical World Around You", when I thought that it would make a good mod. It will feature all the faeries* from "The Spiderwick Chronicles", as well as some that aren't, like vampires, werewolves, and ghouls (like zombies, but fast, and with batlike faces).
*Note: "Faerie", in "The Spiderwick Chronicles", refers not only to sprites and pixies, but also creatures like giants, elves, dragons, unicorns, gargoyles, griffins, etc.
To see faeries, you need a seeing stone, which is made by putting a ring of cobblestone in a crafting table. The seeing stone will send you to the Invisible World, which is exactly like the overworld, except that the days pass by more quickly.