With today's xdg-app 0.5 release, Alex considers this GNOME sandboxing tech to be feature complete for making it possible for third-parties to create and distribute applications that work across distributions. With this sandboxing, these packaged applications have minimal access to the host as needed for functionality.
For future XDG-App releases, Larsson's focus will be on sandboxing and portal aspects of this project. More details on XDG-App 0.5, which has already been packaged for Fedora 23, via this blog post. If this happens to be your first time hearing about XDG-App, learn more via this GNOME Wiki page.