It'd be really nice if you could allow users to create local drafts, of small documents, for instance, and save them locally for free with Firestore.

This way you wouldn't have to coordinate Firestore and its rules with some other database and client code nor would you have to choose the simpler option of getting charged for writes and reads on remote database documents that really only one user is supposed to write and read.

Please note that while you could use the realtime database to save drafts, 1 it's not always easy to figure out whether that'd actually be cheaper, 2 it'd still be cheaper, even with the realtime database, to save drafts only locally for free and then only getting charged for the backup on the remote database.