The system settings need a ton of improvements and some reorganization.
For me personally this has a high priority.
The are so many inconsistencies and issues.
Currently there is a mixture of styles, behaviours and frameworks used for KCMs and they can be very confusing, which is a big problem.
We should port the system settings and every KCM to Kirigami/QML.
We can start by making the two system settings hubs/start screens/layouts that are used now in QML/Kirigami: Icon View and Sidebar View (drop the old and ugly tree view). After that we can port, reorganize and improve every KCM.