Ether fluid

Ether (or Aether) is conceptualized as a "material that fills the region of the universe beyond the terrestrial sphere [...] In the late 19th century, physicists postulated that aether permeated space, providing a medium through which light could travel in a vacuum" (Wikipedia).

That said, it would be cool to bring this failed idea to explain the universe into the Minecraft, in a form of a fluid that fills every "empty" block of a new dimension. However, inside this fluid you can't breathe or swim, and it fills every place as you break blocks, essentially acting as a "air block" that suffocates the player and non-undead mobs.

The entrance of this dimension could be ether-free, as a ancient structure. The way to survive in the ether would be with respiration enchantments and potions (maybe renaming the water breathing effect?).

To consider: The player could fill glass bottles with this fluid and make new weird or terrifying potions that are impossible to brew using water.

To consider: Even though you can't swim in this fluid, it could act slowing player or entities falling speed, while increasing Happy Ghasts's movement speed.

To consider: Elytras could not work in the ether or need a specific enchantment.