Floating End Castles

These would include a large variety of castles, with ranging themes, difficulties, sizes, and types of treasure. They would generate floating on end clouds, which are made of a block with a texture like floating enderman particles that has the unique property that it and any blocks connected to it are disconnected from the Minecraft grid, and can move in any horizontal direction, like one giant entity. The castles would generate in the end like entities, drifting at a random speed in a random direction. If the player doesn't interact with them, they will eventually despawn just like mobs. 

Once inside the castles, there you could find a wide variety of dungeon adventures, maybe with whole new types of mobs, or simply a free loot chest. The in-game generation would provide a constant source of new adventures, and some rarer ones could contain items not found anywhere else. The fact that they are constantly moving would make an elytra necessary to safely reach them and leave them.