The company I work for uses Box a fair amount (which encrypts everywhere) and on a whim I started digging into their documentation to see if they offered a WebDAV service, and, well, they do! - Hypertext
Now that's a service I would love Dropbox to have. It would solve the selective sync issue I'm having where stuff's that not cached on my Mac is not accessible for apps to edit.
I used to have a File Transporter, and their mounted online storage was so useful to have. There's always DropDAV but somehow I don't trust a third party to have access to my data. For security and privacy reasons for one, because I don't know who's' behind DropDAV, and file integrity otherwise. Since this service is a hack on top of Dropbox, I presume that data corruption is always possible.
But it's worth looking into.