Looks neat! This sounds very similar to the concept of unhosted.org[1], where the user provides their own data storage location. Being based in Europe, have you had any interaction with the unhosted team or their remoteStorage.js[2] library? It seems like they are trying to integrate in Dropbox/GoogleDrive into that library, too.
Also, you mention encryption in the Example Applications section. I've been working on a similar concept of having a client-side javascript library that creates an encrypted filesystem on Dropbox/Google Drive, called Bring Your Own Filesystem[3]. The most interesting thing with the system is that these storage locations allow for public files, which means you can set up end-to-end encrypted communication between two Dropbox accounts[4]. I'd love to learn more about your implementation of the encryption layer and how you plan to develop that further. What plans do you have for privacy/encryption?
[1] - https://unhosted.org/
[2] - https://github.com/remotestorage/remotestorage.js