The Rediscovered Mod adds a bunch of blocks, mobs, and items that were either concepts or only around briefly in Minecraft history.
One of the biggest additions to the realm is the Sky dimension. It was originally planned to be put in the game but got scrapped in place of The End. The fun part is how you get there. All you need is a bed and, as if it were a dream, you will randomly be transported there upon sleeping. You are then brought to a small house in the Sky Dimension. You can either fall back down to your own bed or search for the Pigman Village to find the Dream Pillow, which will allow you to travel back and forth whenever you want.
Download here: Rediscovered Mod