The Aether

You use glow stone and build it like a nether portal, then you use a water bucket and use it like flint and steal to make the Aether portal. Once you enter inside, it's going to be bright, sky-high realm full of floating islands, new challenges, and unique biomes. It won't be all lava and fire like the nether, but it would be full of sunlight with fluffy clouds, crystal structures that might provide new crystal blocks, and rare items that gives motivation for deeper exploration. Players may face mobs (both friendly and hostile,) along sky dungeons and puzzles that rewards the player for creativity. The mobs should probably be something like "Happy Ghast" already inside that dimension, or probably a new elytra, something like "Glider" or new tools and armor that uses those crystal blocks, and maybe that could be another spot where players can find the air boost item or something like that. Players can also fall or jump off into the sky that will take them back to the overworld dimension, and when they fall back, they should land on the ground without fall damage.