I rather think the series ends with a reorganization of Harry's world- Mab is preparing to retire by preparing a new Guardian.
Things I have thought may happen:
1) Demonreach is used to imprison/rehabilitate warlocks and bleed off their "taint"- possibly in conjunction with the Blackstaff
2) Harry will use the Blackstaff and the circle on the island to "nom" him some dark god power without dark god darkness to rebuild the outer gates
3) His fatherhood will be important to his motivation to do this.
4) Mab and Titania will leave the world as they entered it- together
5) The White Council will become an educational institution, not a political one.
6) We will find out that Langtry has the world's most amazing bottle cap collection, and that old wizards have been using that phrase in a
completely different way than young ones heard it
7) The Za Lord will become an actual lord of Faerie, and this will make Harry immortal (like the Erlking or Kringle) on its own- and this is what Mab was referencing in
Battle Ground. Related: The dewdrop faeries actually have a form of intellectus.
Martha Liberty freaked out in Battle Ground because she knows #7. I can see a fun plotline where she knew there was this new Wyldfae mantle forming around someone called "the Za Lord," and that it was dominion over the Little Folk, but did
not know it was Harry. She thought it was Toot.
More later. Out of time.