Don't think of it as starting with the "usability" part. This is starting with the "user" part. What do your users want? What do they need? Choosing the users t

Don't think of it as starting with the "usability" part. This is starting with the "user" part. What do your users want? What do they need? Choosing the users that you want to support and what features they need will drive what you need the backend to do and thus the underlying data-structures needed.