It would be really nice to be able to add external media files to the media library.
Of course it is already possible to play music from an external url by using the media_player.play_media service.
However it would be much nicer to be able to see this media in the media browser and having the ability to pick which media player you want to play it on.
One use case for it is playing radio, a lot of internet radio gets played by streaming mp3 files. Thus you are not able to just download it to the SD card and play it from there.
So having the option to add a media url and upload/add an image url to the media browser would be amazing.