As I have written previously, the activities manager service now supports creating private activities.
But it is not exposed in the UI at the moment.
Now, that is not the only problem that the current settings module for Activities has. A few things have improved in the previous few versions, but there is always more room for improvement.
One of the common complaints was that the activities settings module does not actually allow you to create, modify or delete activities.
Both issues will be fixed in the next release of KActivities framework.
In our discussion on the Visual Design Group forum [1], we proposed to group every setting which can be activity specific in the same place, to improve discoverability of the possibilities of activities. This KCM could be the basis for it.
For instance, for power management profiles, you would define in the appropriate KCM the default profile. Then, in the activity manager, you can choose either to use the default, or to overide it with an activity specific setting. If you open the power management KCM in an activity which as an overriden setting, then a warning would be displayed and would link you to the activity manager.
For the UI, I would rather use a List-Detail pattern [2], with Activities in a list on the left, and every setting (including privacy) on the right.
[1] https://forum.kde.org/viewtopic.php?f=285&t=126253 [2]https://techbase.kde.org/Projects/Usability/HIG/Patterns/NavigationPatterns#Patterns_for_a_flat_content_structure