This should be an option, but only at the folder level, and preferably shared only with specific apps.
We do not want to give Google and Android apps free access to all of our content. Some apps ask for ridiculous permissions levels and make no bones about mining and stealing user data.
Why not incorporate some of the UI and code from Groove music player into OneDrive to enable background and offline music playback? Music and photo storage are important for your subscription service.
You should incorporate the Groove Music app's user interface for the music folder of OneDrive (or alternatively don't kill support for Groove Music player). Music and photos are the chief reason anyone uses OneDrive for personal purposes.