Rahul, a query for you though-- sorry if you're the wrong person to be asking. Under Apper in the KDE spin there's a set of config options to enable checking for distribution upgrades, not just package upgrades. Any possibility of one day actually -using- that feature, using fedup as the backend? Like just have packagekit tell the user a week or so after release "Hey! Good news! The next fedora version is out! Click me if you want to upgrade!" and its basically just a graphical frontend to "fedup -n (`$RELEASE`+1)" Sorry if the bash syntax is wrong but hopefully you get my point :P I don't know if the Gnome Software app has a similar config option, but it seems like a good use of it if it does.
GNOME PackageKit actually used to call preupgrade in Fedora to do what you are suggesting. I don't think they have hooked up either GNOME Software or Apper with fedup though. Perhaps you can file a RFE in Red Hat bugzila against apper? I am not involved in those projects directly other than contributing some documentation for GNOME PackageKit.