Yeah, the OS should already treat any archive-type file as a filesystem automatically. So you can "cd" into a .zip, .tar or .ext4 file (no mount -o loop necessary). Applications have some support for this (MC and emacs in UNIX or Windows Explorer), but it should really be in the OS.
That would be very interesting indeed. Even Plan 9 requires you to mount tar files with a special command instead of simply typing "cd file.tar".
The Plan 9 developers were conservative about preserving the semantics of commands like "cd", so that the system did not diverge too greatly from Unix. Instead, the equivalent of double-clicking on a file in Windows Explorer is handled by right clicking on a file in Acme. Acme sends the filename to something called the "Plumber", which then uses rules to decide what to do with the file:
