My bet is a paranet heel turn in the later books. Picture this: the white council breaks down over time. Harry has personally saved their asses many times, now they've turned their back on him, and Jim has thoroughly shown what happens to groups that can't adapt when they don't have a protagonist on their side. The paranet starts to become the premier magical organization in the world, since it accepts all magical types, including presumably increasing numbers of former white council refugees. These wizards train up the minor talents, making an organization of literally thousands of increasingly competent wizards, backed up by a support base fully integrated with modern tech and society. It's a nice big support structure of suitably grand scale, just in time for the BAT. But Jim pulls a Wheel of Time, and has the Paranet go full Black Tower, with our protagonist's baby suddenly turning evil, because Elaine (probably a thrall herself, her actions in Dead Beat are not logically consistent) has enthralled or influenced many commanding members of the paranet, just like peabody did with the council. They do work for the same guy after all. Plus Elaine is demomstrated using mental magic twice that I can think of, so it's not outside her skills.
Jim has repeatedly says his goal as an author is to hurt the reader, ang having Harry's big ray of hope in the paranet suddenly turn to the dark side right aa the BAT kicks off would be a great (and foreshadowed) gut punch.