I've always liked the concept of a logical filesystem consisting of several other filesystems treated as layers, where accessing a file would access each layer in order until it finds one with the file. This would be especially useful for a system where the majority of the data is read-only, with a small amount of writeable information.

Such a system would be very useful for customisation of Knoppix beyond home directory configuration stuff. If a base Knoppix FS was layered with a writeable FS, such as a Memory Stick, then you could transparently change settings in any directory, and even more useful, install new packages. If Knoppix has everything you need except a few packages and licenses, which aren't included for some reason (license, space restrictions, popularity, personal code), then you could install it. The altered files would be on the memory stick, and new modules and upgrades would be an apt-get away.

Unfortunately, I've yet to find a well-supported working implementation of such a layered FS (although I haven't searched lately), which is a shame, as it could have a myriad of uses - layering a base NFS Root with local customisations, using CD bases such as Knoppix, even on a local HDD, keeping a partition Read-only so you can always roll back to a known good system.

My own code capabilities, and free time don't allow me to write such a FS - although I could do it in Perl some time.