It would be great if the Subsonic server could be set up with cloud-based (eg Dropbox, OneDrive) endpoints as another source of files. This would enable a Subsonic server to be used as a personal gateway to local as well as cloud-based media.
The user could use any normal Subsonic client, as the Subsonic server does the work of storing login tokens and streaming (and caching) files from a cloud-based store. Bonus points of the server is smart enough to use local copies of cloud files if available to avoid hitting the cloud.
My particular use case is that I have a machine with a lot of music stored locally that I access via Subsonic. Recently, I've signed up for OneDrive and now all my music also lives there (that is to say, I moved my Music folder to be within the OneDrive folder). I'd like to keep more of my data "online-only" rather than storing it on my desktop computer. Microsoft has an app called "Groove Music" which does a decent job of playing music directly from OneDrive, but it is only for music, has limited file format compatibility and only works for a certain amount of data.
p.s., I've been a happy paid user of Subsonic for at least 5 years. So happy that the product continues to evolve and is so dependable. Takk Sindre!