Mounting problem should be the main issue in the proyect, but I've no experience working with FUSE and whatever. An addition I would include should be a kind of link on the cmg's header.
For example:
An uncompressed version of the app icon.
The complete app name (the file's name isn't very descriptive many times).
A list of mime types the app supports (interesting for automatic setup of KDE or GNOME)
A way to execute application files/documents in the cmg file, for example, opening a PDF file using the Acrobat Reader in cmg mode. A script than mounts reader and opens the pdf doc on it. Of course this should be for AppRun