These are early attempts at recreating past ideas for a unified system tray popup. Also including - critically - the sidebar in all areas
Something that I'm not particularly fond of the widget popups are the check boxes. In earlier versions of plasma they were huge, now they're smaller and more appropriately looking but I can't help but feel they add clutter in a way.
What do you think of a possibility of them being toggle buttons (so they can be pressed down when active) or to have some sort of active indicator.
Below is something I quickly made in KolourPaint to demonstrate what I'm thinking about. You can also vizualize them as pressed buttons. My paint skills are not sufficient for that one.