It would be really nice to import JSON (or possibly XML?) from other places to make a quick and dirty database.

Also, it would save more work if I could copy items and categories from one database to another.

If this is a minority opinion, you can disregard this thread.

That's a great idea. I've got a fair amount of data in Excel, so an import from Excel or CSV (or XML, JSON, etc) would be great for me as well. 

When this app is billed as a replacement for spreadsheets, an easy way to transfer data into it can only help users get the best out of it.