I agree with beckfield – unless there’s some complexity how you share those libraries, collections might be an easier way to achieve that.
If you need the subject/class based structure for sharing, you could alternatively consider using a sharling-label (English, Geography…) and let Plex dynamically create a smart collection based on that same label (1 stone, 2 birds 
)
That being said, I believe your suggestion is pretty close to what’s been requested some years before – though that never got much attention/support since then.
