Bazil[1] has similar goals, but wasn't ready enough for me.
I do very much like sibling /u/pritambaral's suggestion of OwnCloud support. I tried OwnCloud briefly in the past, but ended up switching away. A feature like this might get me to try it again and stay.
[0]: https://github.com/burkemw3/syncthingfuse [1]: https://bazil.org/
Yes, because everyone knows it would be amazing to have this kind of functionality just work for everyone, regardless of their personal OS and technology/device choices.
A couple days ago I contributed https://github.com/DanielDent/git-annex-remote-rclone to git-annex, so in theory you could use Dropbox as a backend for git-annex if you wanted to (I haven't personally tried it with Dropbox).