What I've really always wanted, and still never seen, is "Hierarchical Storage Management with the canonical copies of everything in the cloud, and a bounded-size MRU cache on all my local devices, but where updates to the cached files are taken as updates to the canonical version."
This way, you'd be able to have petabytes of stuff seemingly "synced" to your local disk; stuff would just be (much) slower to access the first time, or if you hadn't used it in a while. (Sort of like Apple's "fusion drives", but with the spinning disk very, very far away.)
It's almost like a FUSE-mounted WebDAV share or somesuch, but where you interact with it through its local cache on your real filesystem, rather than by your OS making its read/write requests to the server itself.
This way, you'd be able to have petabytes of stuff seemingly "synced" to your local disk; stuff would just be (much) slower to access the first time, or if you hadn't used it in a while. (Sort of like Apple's "fusion drives", but with the spinning disk very, very far away.)
It's almost like a FUSE-mounted WebDAV share or somesuch, but where you interact with it through its local cache on your real filesystem, rather than by your OS making its read/write requests to the server itself.