Being able to send another user a notification using the API would be nice. It would unlock the ability to experiment with different sorts of workflows without having to change MediaWiki to support more notification types.
Moreover, I see lots of talk page sections that are just notices and not intended to be responded to. These include newsletters, notices about discussions ("An article you started has been nominated for deletion"), and other mass messages ("Upcoming meetup in your area"). These aren't discussions, so they shouldn't have to go on user talk pages.
For users who prefer the old workflow, there could be an option to convert all notifications sent using this API to talk page sections. Putting this behind a permission and limiting it to admins, for now, will probably address most abuse concerns.