A tool for migrating to another version or a reinstallation of Ubuntu by collecting information from the user and copying information into a .deb, preferably a meta-package.
First the tool would ask what information the user wants to store into the .deb, desktop files, settings in /etc, list of installed programs (which would serve as a meta-package), Gnome panel/nautilus/whatever settings.
The application would then copy the relevant data and build a .deb
The user could later use this deb and execute it normally to quickly restore users and groups, network settings, desktop files, drivers, package selection, and anything else the application could collect for them.
Of course, without a separate application on the target computer, this method couldn't be practically used to remove unwanted packages.
The usefulness of this is limited, but when it can be useful it can be an incredible convenience.
The information can be collected through a drop list, the user checks what preset information they want to save. One list can be populated by the list of installed packages, one list can be populated by the contents of their /etc, some lists can be custom built to pull gconf settings and populate the list of which ones to be saved.