I think this is one big area of improvement all around that needs work. From better integration in iOS 11 (file browser) to using a NAS to sync directly with. For istance if I want to work and edit files located on my NAS. Weather it stores a temp copy locally when editing but keeps everything on the NAS. Or be able to fetch and re upload when done. Obviously network speeds are important here. Most of my files I work with are on my synology.
I don’t see just one solution here. I do think this will help a lot of users though. I just think file management needs more work.