 Starting from Plasma 5.22, there will be 3 new options in the panel settings. You will be able to have an OPAK panel, a transparent one and an adaptive one. The adaptive option will make the panel OPAK only when a window is maximized and transparent when you demaximize it, as you can see. This allows us for a cool elementary OS or Ubuntu like setup. It is fairly easy to enable this in your personal theme. Just add adaptive transparency enable equals true to your metadata file. You can also customize the look of the panel when a window is maximized, like making a threat for some reason. You can also give different panel, different settings such as an OPAK one and an adaptive one. Finally, if you hate transparency, making a panel OPAK will also make all widgets OPAK. This should be able to cover all use cases.