As much as the book is packed full of ideas, the book alone doesn't give you everything you need to get the most out of it. Reading GEB has often been a solitary experience, with readers having to go look up the music, the cultural references, the details of the math, and the ways these fields of knowledge have changed in the decades since it was written.
You could imagine a supplement to GEB that contains everything the book doesn't. A place to find "notes in the margin" written by other people sharing the experience of reading the book. That's what this wiki could be.