When designing a system you always need to start top down.
Too many times I see a developer sit down an immediately start from the database.
Start from the UI or API. Just like we have UX for UI, you should be taking experience into account when designing an API.
It's a known programming adage that you should start by getting data structures right, and proper algorithms will follow. In my experience, the shape of data and ways you work with it have so huge impact on what must be presented on the screen that I'm starting to feel it might be best to delay UI/UX work until you have all that figured out.