It's been over a decade since the announcement of the unreleased "Moon" chapter, also known as island6 or Chapter 0. I have an idea, maybe it will return when the 15th anniversary of World of Goo rolls around (Which is this year!), like with the Ho-Ho Holiday expansion for Little Inferno. Along with this I've heard something about downloadable levels in the Wiiware version, maybe these could also be implemented? There were definitely plans for user-created content, as evident by strings in some beta builds mentioning a level, ball, and even chapter (formerly world) editor, plus a "Share Creations" and "Download Creations" button, implying a site or world in game where you could share your creations or download them.
Here's another oddity, 3D! Yes, 2D BOY was intending to add an option to make the levels 3D via Microsoft's Direct3D. You can't even access this in game, so it's pretty much useless.
OHHH also, HAPPY 15TH BIRTHDAY, WORLD OF GOO!
:)

~Your friend, Big Tennis and The Sign Painter