First thing that came to my mind when I read this was IPFS. Implementation would be hairy. You’d have to add some encryption. If it was planned carefully, it could be super cool. You could have all the sync and maybe some of the sharing stuff even when connectivity to the server was bad or non-existent. First step would probably be to write a general dav client that also supports ipfs, and then, if you wanted more integration (sharing, etc.) look at the NC specific bits.