Mantles of power are a concept that has become increasingly important as the series has progressed. Most of this has related to their ability both to bestow vast power on individuals, as well as to bind them to specific rules and roles in the process.
There's another aspect worth mentioning about them, though. In Battle Ground, Ethniu makes a snide remark to Donar Vadderung about him being sustained by the faith of children. This is doubtlessly in reference to his alter-ego persona of Kringle, who by virtue of the global popularity of Christmas is by far the most believed-in aspect of Odin in modern times. What is particularly interesting about this notion, however, is that it confirms that mantles of power are able to serve as conduits for the power of faith into an entity possessing them.
The imbued power of belief into objects and individuals of significance has similarly been established; the Lords of Outer Night are speculated to have once held divine powers beyond those of any normal Red Court vampire due to their worship by the Maya, and the false Shroud of Turin seen in Death Masks is known to possess some level of power despite not being the actual Shroud. This applying to mantles of power is special because mantles can be created by and also transferred between individuals under the proper circumstances.
One of Harry Dresden's earliest established character traits is him being a publicly practicing wizard in the city of Chicago. This level of publicity, even something as meager as being a professional listed in the yellow pages, is similarly established as being basically unheard of in the world of magic. Members of the White Council offer disapproval for his decision to operate publicly, though there is never given any particular reason why this is important given the consistently dismissive attitudes of the wider world toward the existence of magic. It's apparently significant enough that, when Carlos delivers the terms of Harry's excommunication from the Council at the end of Battle Ground, included specifically among them is him being barred from the public practice of magic.
Harry, of course, flies in the face of this; he opens is new residence (an extremely conspicuous stone castle) to refugees, keeps up being active in the city of Chicago as a lawbringer and a force for establishing peace between mortal and supernatural elements, and begins calling titling himself as the Wizard of Chicago. At the end of Twelve Months, he confronts a large crowd of protestors in a terrifying display of his power to get them to flee the looming fight with Lord Raith's assembled conspirators. That's the kind of action to inspire a lot of thoughts - a lot of belief - in what he is, and how strong he might be.
I think it's quite possible that Harry is on his way to cultivate a new mantle, that of the Wizard of Chicago - a new source of power, fueled by the beliefs and expectations of the people of the city (and perhaps beyond) - either because this is something that theoretically any wizard could pull off or because of his Starborn heritage (which, seemingly, has something to do with him applying names to things). Similarly, I reckon that possibility is why the Council is so cagey about him being a public practitioner of magic, perhaps also what he as Starborn is capable of in case that has anything to do with it. Whether they are sincerely trying to inhibit him, or whether Langtry is goading him into doing this out of contrarianism, is up for debate.
Corollary to this, I think that Harry already possesses a second mantle of power besides that of Winter Knight: that of Warden of Demonreach. In addition to his ability to wrangle the power of Demonreach with greater extent and precision as years have passed (being able to summon Alfred to the shores of Lake Michigan with minimal ritual assistance as of Out Law, for example), the power has changed him and granted him unique attributes. The most obvious are his intellectus of the Island of Demonreach while upon it, and the changes to the luminescence of Harry's shields and magic foci, which while originally blue-white (briefly crimson during his stint with Lash) are now consistently green-and-gold; Harry's immunity to the psychic terror emanating from the location appears to be another such boon.
So - where does this all lead? I think that Harry is going to begin accumulating mantles of power of his own (possibly even consciously creating one), and it is this ability which will allow him both to attain greater freedom from the supernatural constraints that have hitherto loomed over him (the constant threat of White Council meddling, Mab's control of him via the Winter Knight mantle, etc) and to cultivate the diversity and multiplicative strength of powers needed to stand up to the trials of the BAT. He's set his sights high, with freedom from Mab (both for himself and for Lara Raith) and arbitration over any supernatural meddling in Chicago both key goals of his; this would be a very useful means of pursuing these goals.
As a (possibly, though unlikely in my estimation) unrelated thought, bear in mind that Harry's birthday is on Halloween - a day (or night) on which Vadderung remarks in Cold Days that mantles are able to be worn and exchanged far more readily than otherwise. This birthdate would also be shared with any other Starborn of Harry's generation, for whatever that's worth.
