Data can be added to the model in OnModelCreating or similar which, on other providers, including Cosmos, will then be inserted into the database when EnsureCreated is called. We should consider doing the same.

Model data is generally frowned on by the EF Team--it's something we wished we hadn't built. But that is because of the way it interacts with migrations. I can't think of any similar issues in the document case. However, we do need to be clear how EnsureCreated is intended to be used with MongoDB.