Currently, the "Upload JSON" button assumes that you're uploading a document with the exact structure that's already in the catalog - presumably to update the values. However, if you're creating a new project or doing a big refactoring of your data structure, it would be nice to piece together a catalog from existing parts to create a new catalog.
Side note: This would also allow for a workflow where you can create and manage your catalog collaboratively with your design team using Google Sheets (see Rich Joslin's Google-PlayFab bridge), and just use the "Upload JSON" button when you want to launch a new catalog.
We would love to know what you need. Submit your ideas and upvote others to help us prioritize.