For me it's quite logical step to split Apple Music from iTunes and discontinue iTunes if they release an official "syncing" app for macOS. iTunes made sense in iPod era, but ever since there was an iOS + App Store, it didn't make sense for me to have Music library + Music store + syncing / backup for iOS in one single app.
So I think there should be standalone Music app and something like Files / Cloud sync app (on macOS) for everything you need to manage files / backups between macOS and iOS. I can't imagine the real implementation of it, but I hope Apple will make it really straightforward. And of course they should provide you free iCloud Drive space in size of the device you are using, not those ridiculous 5gb.
So I think there should be standalone Music app and something like Files / Cloud sync app (on macOS) for everything you need to manage files / backups between macOS and iOS. I can't imagine the real implementation of it, but I hope Apple will make it really straightforward. And of course they should provide you free iCloud Drive space in size of the device you are using, not those ridiculous 5gb.