What I would really find valuable is a translucent
filesystem, i.e. a way to super-pose a directory over
another, with all write operations being redirected to
the "top-level" directory via a copy-on-write
mechanism. Currently there is an LKM implementation
(translucency.sf.net), but it is far from perfect,
since it messes up symlinks and does not support
deletion of files. Moreover its behaviour when it comes
to file permissions is far from perfect.
Using such a filesystem would be valuable for people
who want to perform tests without trashing their
system: the root (/) would be used as a base, with
another dir placed on top of it with the translucency
FS, protecting the root FS fron write operations. This
has the additional advantage that all changes to the FS
are reflected to the "top" directory.
I believe that such a filesystem can be easily
implemented in LUFS, possibly based on localfs,
although with a significantly lower speed than a pure
LKM version (which is much harder to create, though).