I believe most plugins expose their GUI only as an X window that needs to be nested in an application window, so any change here would require reimplementing the GUI of a lot of plugins.

But even if we do want to take this hurdle, is it even possible yet? What interfaces are available to do this on Wayland and embed a plugin-drawn GUI in a GTK 3, GTK 4, Qt 5 and/or Qt 6 application?

My guess is we still need a new spec that would probably revolve around OpenGL/EGL or Vulkan/WSI, but I'm not sure, and there's also the question of how input events are delivered.