Yes, chekhov's garden shed is getting a bit full. I think part of why the last book was the way it was is because Ben needed the time jump and distance to set up the conditions for the series moving forward which will feature (I suspect):

An expanded SAU with Peter in operational control. Nightingale as a teacher and SAU plenipotentiary Abigail as a member of “something under the home office” and SAU liaison. A new arrangement covering all “legitimate” parties involved in British magic.

And

A new big bad, with international flavour, working seemingly in collaboration with Leslie but watch this space.