Currently the only way to toggle fullscreen of an application is by right clicking on the titlebar then going to more actions and finally enabling fullscreen. If there were a way to add a button to the titlebar itself which would enable fullscreen, allowing for better focus, it would be good and a nice alternative to Mac's own fullscreen option, although without the forced tiling mode. It would not replace the maximize feature but only be an alternative disabled by default but able to drag to the titlebar on Titlebar Buttons options.
The issue is that mac has a completely different fullscreen mode workflow. If kwin gains such a workflow too so you can still access the titlebar somehow (this in its turn will require major changes on kwin side, and I'm not sure we will ever implement it), we will add a fullscreen mode button. But otherwise, it'll be only one way, and it'll provide poor user experience.