RIP Firefox OS. :/ It might be a little challenging to base on a platform with not a lot of future support.
I use EtherCalc as my primary spreadsheet editor, and I can definitely agree it's UI is not it's strongest feature, though, it's way better than having nothing. I definitely noticed how clean FireText looks and feels compared to Etherpad, which is my main text editor.
Regarding export/import, I think the biggest thing (and Sandstorm doesn't do this well either currently) is an easy way to do that in bulk. For example, I'd want to export all of my documents in one batch, so it's something I can quickly do regularly as a backup. And for import, I'd want to see something someday like where I log into an OAuth for Google Drive and it auto-imports all my crud. Consider the import flow for most email services: You log into your old provider, and it uses IMAP to pull all your old mail in. One and done.
That's not to suggest I think this is a quick or easy thing to implement, just food for thought.