It would be very useful to add a "Scale Display" control to the plasmoid. Often, when you switch monitor, you need to adjust scaling factor, either because the external monitor has a different DPI, or because you want to send output to a projector for classroom use, where you need large fonts. It would be nice to alter scaling factor quickly.
Interesting idea.
Everything else in the applet is currently global, rather than per-screen. But on Wayland, each screen has its own scale factor, so we would need to add a screen chooser UI to the applet. I think this would be quite hard to stuff in there in a way that felt right. So I don't think we'll be able to do this, sorry!