Check out what nostr:npub1ve8qwrlztemdmnh62jffcr0y9m9dpqgqjdg8ufx7gc3qw5wdk74qyv9ka6 did with their Nostr integration. All users can link an npub and use it as a login method. (They also have a NIP-05 and a Lightning wallet with some pretty nifty features!)