This biome would be a dimension that you can access through a portal (like a nether portal in terms of building and style) that has both very dark and very light elements to it.
It has an enderman TYPE mob (but light and obviously not the same as an enderman), as well as friendly flying mobs and hopefully multiple other types of mobs, both friendly and hostile.
It has lots of greek style royal structures in it, as well as ruins.
It consists of lots of floating islands and new blocks and new trees and different ores (which give way to being able to make new powerful weapons and armor).
It has a boss that you can fight that may or may not be stronger than the ender dragon, but slightly easier to defeat. Defeating this Mystic Boss will earn you free rein of the entire dimension and the allegiance of non-hostile mobs who will fight with you, and the ability to tame hostiles. It will also earn the player lots of experience and wonderful new items, weapons (that you can only earn by defeating the boss) and armor.
----Still an idea, but I and my friends would love to see this implemented sometime in Minecraft.