One of the great things about dropbox is that you don't need to predict. You can just look at their feature request list and see what's on top.<p><a href="https://www.dropbox.com/votebox" rel="nofollow">https://www.dropbox.com/votebox</a> (API is nr 9)<p>Although I'm not sure what file system for the web means. I believe dropbox itself stores files in S3.