Right so this is a small project to slightly improve and make system settings FEEL more native (as opposed to just BE more native) in Plasma.
The main issue being of course that tree-view feels alien to Plasma 5, the Icon Grid is hiding further settings behind a front page and that KCM's are written in QtWidgets. The latter is key as that means that we need to rewrite every single KCM which we obviously do not have manpower to do.
We could edit the tree-view but that would mean even more work than the current solution as the code is ... a bit of a mess... but anyone looking for a challenge who want to implement the visually small changes to the tree-view just "ping for mockups"
Most importantly - all this will, in a utopian future when all KCM's are rewritten in QML, reorganized and redesigned be wasted work. This meaning that we need to use the cheapest solution available to not waste man hours entirely on something that will be rewritten from scratch.
An initial idea was to create a secondary side bar on the right to the main list of Settings to display each sub-section - but this proved problematic as certain KCM's did not display correctly and would require more work than justifiable. This was therefor rejected in lieu of the tab-bar on top. Posting the initial wireframes for that below as well! It would also require a small reorganisation of sections to not create a massive list on the left hand side which would be confusing at best. If anyone want to give that a go of course - bring it on!
The problems with tab-bar on top (a sudden secondary system of organisation) are therefor justified as far as we could see.
Finally there is one thing, an edit to KCM's that would be relevant and that is the movement of all "Get Hot New Stuff" buttons to the same location across all KCM's - this though is beyond this small projects scope although it would be hugely beneficial to the UI of System Settings.