* p7zip, being a command-line tool, is something "normal users" should never need. If Archive Manager needs it as a plugin, it can install it with PackageKit if it's not detected, or it should be a dependency in the RPM if Archive Manager doesn't support that. Hunting down a magic package name to install is not an acceptable user experience, and not something we should encourage or optimize for. Alternatively, it could include an appdata file so that it's listed as an Archive Manager plugin in GNOME Software, but I think that would be non-ideal in this case, since users should never have to worry about having the right packages installed to unzip an archive.

So this means that a default Workstation installation should have a large number of non-graphical programs installed by default, or that the various applications will be compiled with every possible dependency, e.g. Archive Manager will pull in every possible compression/decompression back end?

One more note here: if you try to run a command line program that isn't installed, PackageKit will search for it in all enabled repos, offer to install it for you if found, and then run the command. It's really a nice user experience, and it's what I use to install executables that I don't have. This only helps for executables, of course, and it doesn't help users who first attempt to find the package some other way.