I mentioned when I started posting that I had planned this fic in part in response to the dearth I perceived of fics where the travelers meet alters of their own selves. Another very common theme I noticed when reading a lot of fics in this fandom is fairy tale themes, and so each of the worlds they visit here is (vaguely) modeled after a fairy tale as well. The first one, of course, is The Little Mermaid.
The Missing Worlds - Water World II
Rating: PG-13
Spoilers: This story is set in the uncertain period after Piffle World, but before Recourt; in other words, in the same never-neverland that the second season of the anime mostly took place in. This means that Fai has not yet learned to whistle, Syaoran is still the original Syaoran, and nobody knows anything about Kurogane's childhood.
Summary: The travelers visit the Emerald City, and go off to see a wizard. Wait, wrong story.
~to be continued...
Author's Notes:
In addition to the breathable air of the world of Chelestra - which also has the magic-blocking properties Mokona describes here - I also modeled this world partly on the underwater zone of Vashj'ir.