I'm a big fan of Minecraft Lore, and I think this idea adds to it. The End Dimension, as theorized by many, is the place where an ancient race of builders were trapped after a series of events, and gradually morphed into Endermen. My theory starts here: Somewhere between the morphing process, they gathered enough obsidian to try making Nether portals in the End, and managed to light it with soul-fire, but it lead to a new dimension: a vertical dimension that already had another race of builders. Because of that, they ended up in war against each other, but they were outmatched due to the unique properties of the dimension:
The Ruins dimension is a 4096 x 4096 dimension encased with bedrock on 4 sides, that extends infinitely (or upto 60mil blocks) in the vertical directions. It randomly changes gravitational direction between up and down. Different players / entities can be in opposite orientations, and even the same player / entity can be in opposite orientations in different times. This will force building structures to become very creative. This dimension will also always be bright as day.
Back to the lore:
The structures of this dimension will often be damaged, with most of them being usable in both orientations, made of blocks found in this dimension; there will be structures primarily usable in singular orientations, mostly made of blocks from the End.