Add a possibility to edit protocols.  e.g. ed2k (eMule), feed (RSS), irc (IRC) magnet (BitTorrent), mailto (Email), xmpp (Jabber) etc.

In such case, I guess, it would be possible to change a default Email client with both exo-preferred-applications and xfce4-mime-settings.

It would be much comfortable using xfce4-mime-settings over a buggy XUL web browser that sometimes decides, by its own, a different application than the default one.