Part of where this is coming from is that I see harry as steadily moving up the ladder in the Court, or at least in how he interacts with them. Once upon a time Lea made him quake in his boots and avoid the NN at all costs. Now he orders her around, they have a much more even/level (gadara style) relationship. Ditto his relationship with the Winter Lady and arguably Erlking, albeit with their own circumstances. Before it's over I think things will have moved beyond Mab, and when Harry starts acting like the Mother's Knight, I could see Mab getting backed into that kind of corner.
You know, Q, I don't think so.
I think Harry is as high as he's going to get, in the Fairy courts. I think with these last 5-7 books he'll beat up more of the courts, but that's because of his current mandates, but I think he'll always be Mab's knight.
Jim has said he didn't ever want Harry to become a big fish in a big pond. Harry is always supposed to be a small fish.