(Dimension) The Above

Getting There

To reach the Above, a player would have to be up above Y level 512 at exactly noon. Thus getting there would require an elytra and rockets. The player would arrive in the Above at the same Y level they were in the overworld, and would have to descend again to reach the terrain of this dimension.

Physics

• All directions look like blue sky during the day, black at night.
• Items don't rise or fall - they just float where they are.
• Gravity is half, but terminal velocity is double.
• 1:16 block ratio with the overworld.
• Water freezes instantly.
• Lava turns to obsidian instantly.
• No void fog. Players take slow suffocation damage just below Y = 0,  lots of suffocation damage far below.

Aether Blocks

The "terrain" of this dimension would largely be composed of structures made of Aether blocks. These blocks look and act like air except under certain conditions, and are usually translucent and intangible even when visible. To get an idea what they do, it might be best to describe some structures made of Aether blocks:

Structures

• Clouds - Slow the player down, appear and disappear with weather. Thick layer of storm clouds near build limit.
• Stars -  Appear only at night. Pure white, produce light. Usually close to the void.
• Sun rays - Form in long, straight, horizontal lines. Visible and active only during the day. Move player faster.
• Moon rays - Work like sun rays, but for the moon. Stronger during full moon, don't appear during new moon.
• Drafts - cover large areas. Never visible. Accelerate player up or down like bubble columns.
• Floating Islands - Not made of Aether blocks. Very rare.

Mobs

Phantoms (the ones from the overworld) would frequently spawn in the Above at night. The idea is that the Above is where they come from, and they sometimes cross over to the overworld 

• Nimbus - shadowy cloud-dwelling mobs that fight using lightning. Spawn whenever it rains, especially during thunderstorms.

Leaving the Above

Players would have several options here:

• Fly back up to y = 512 at noon.
• Sleep in a bed - this would not set your respawn point, but instead you would wake up in your overworld bed.
• Use a Nether portal - this would take you to the Nether. Portals could not take you back to the above, as they would link to the overworld.

If you have an idea for a mob or structure in this dimension, leave a link to your post in the comments so I can link to it here.

Lastly, I give credit to this comment, as the inspiration for this idea.