Mantles are made from the nine we have seen, they do not occur naturally. The only mantle concerning Harry which he owns in on his duster, and over the fireplace in the Castle. Mab owns the Winter Knights ass (does Harry now have a snowflake tramp stamp?)
What is more likely is that Little Chicago inadvertently created an embryo genius loci for Chicago(massively accelerating the natural process) and that Harry bonds with it in the same way he did with Demonreach, so when Harry passes, another wizard can bond with it as the new Wizard of Chicago. Harry had Soulfire after Small Favour so that may have inadvertently kick-started the process, and embryonic Genius Loci escaped the model during the fire (mirroring the Great fire of 1871 which led to major rebuilding) passing into and connecting with the Ley Line network (which Harry modelled in Little Chicago) to draw power.
Demonreach is special and old amongst genius loci, so Chicargo would not be able to gift the same powers to Harry, at least not during his lifetime, but it would for example give Harry a home field advantage in being able to draw more power as he anticipated with the Sanctum invocation involving Demonreach, especially as it will be a big genius loci, whilst operating within Chicago itself, maybe as much as he was able to wield in in Battle Ground.
It is a logical power up given Harry’s recent progression and WOJ about Harry becoming more involved locally in civil affairs, far more than the creation of a Mantle.