I think there's a lot of interest in this topic generally, so I hope this continues to develop. I for one pay for ExpanDrive, and am also aware of Mountain Duck in this space.
In my experience, those options are functional for infrequent operations, but not constant daily interaction, in the way you can with Dropbox for instance. Offline caching is of course one major aspect, but also I presume lots of error handling hidden in the background for smooth operation.
Thank you for your comment. I'm surprised to see such interest in what I'm doing. As this is just a personal hobby, it's difficult to make rapid progress, but I will try to develop it as much as possible.