This is a post on Reddit, not my own theory. I ran across it and found it very interesting. It basically says that Harry has fulfilled one of the conditions of becoming Mab by giving birth to Bonnie.
"You? The new Summer Lady?"
Lilly flushed prettily and nodded.
"I know. I didn't want it, but when Aurora died, the power flowed to the nearest Summer vessel. Usually it would be one of the other Queens, but I had the Knight's power, and it just sort of...plopped in there."
Sooo....we know from Cold Case that at least the Winter Lady's mantle stops from getting pregnant, because she has to remain a Maiden to be the Lady. If she became a mother, she would be the Queen. We also know that the Queens' mantles can transfer to someone holding the Knight's mantle if another Queen isn't around to grab it. We ALSO know that Mab was mortal once ("I was mortal once."), so the Queen's mantle can transfer to a mortal.
I think it's safe to say that Mab is probably going to die during the Big Apocalyptic Trilogy. She's been both built up as a terrifying force and humanized, so narratively this would go a great deal towards showing the scope of the threat and creating drama.
If the Winter Queen must be a mother, and holding the Knight's mantle is enough to qualify you as a vessel, then let's consider for a moment that Harry Dresden is the Winter Knight, he has been raised and trained by Fey for most of his life, and he just gave birth. Yes, Bonnie is a spirit entity, but in the Nevernever that probably counts for more than birthing a real child. Also, being a Starborn and a powerful wizard in his own right could be the edge that the Winter Queen needs to finally tip the scales and push the invasion back.
So that's one of my predictions for the BAT. Queen Harry standing alongside the Knights of the Cross (Butters, Sanya, and Marcone) and Demonreach's denizens against the Outsiders.