The only thing I'd change at the moment would be to make 
  a Cloud backend API so that customers can plug in 
  Dropbox, Google Drive, Box.net etc. with something simple 
  like a URL, separating storage from display etc.

For what it's worth, that's specifically what our differentiation was with OpenPhoto/Trovebox (we support about 7 storage services along with being open source[1]). We've moved on from the consumer space because there didn't seem to be a large enough market for that specifically; separating data storage from application logic.

You can see our Kickstarter from 3 years back[2].

I asked in another post how offering custom S3 buckets was working out for PictureLife. Curious if someone else found a market for that.

[1] https://github.com/photo/frontend

[2] https://www.kickstarter.com/projects/jmathai/openphoto-a-pho...