After several attempts trying to write a new KCM for network configuration and actually not finishing any of them, I decided to start one more time, but this time my goal was to simply transform the old editor into a bit nicer KCM and place it into system settings where this was missing for very long time. You can see my current result below.
This is still same editor as it was existing before as a standalone application, except the list of connections is now written in QML and is similar to the applet we have in systray. I also had to rewrite the editor widget a bit because it’s currently implemented as a dialog with a tabwidget inside where each tab is represented by one setting widget (e.g. Ipv4SettingWidget), For the new KCM we now have ConnectionEditorBase widget doing all the logic behind, like creating specific setting widgets based on connection type and so on. This widget alone doesn’t display anything and you have to actually subclass it and reimplement method taking care of layouting. This allows me to have e.g. ConnectionEditorTabWidget which just subclasses ConnectionEditorBase and reimplements addWidget() method to place setting widgets into QTabWidget. In future we can also simply write a new UI/layout on top ConnectionEditorBase widget and get rid of the tab layout.
Regarding functionality, it should be already almost on par with functionality of the editor. There are still some missing features (like import/export of VPN), but besides that I think everything else is going well. With the new KCM there are also some minor improvements, like you can now reset your not-saved changes you made to a connection. My plan is to get this into Plasma 5.9 which is supposed to be released in january so I still have plenty of time to finish missing features and address issues I made during this transition and of course time to take your comments into account and make this KCM as most usable for everyone I can :).