The transformation of the Darksign at the beginning of the trailer from the typical red flame into the white flame heavily suggests a connection with the "Usurp the Flame" ending of the main game. In the last part of the game, the sun has been eclipsed by something (capital D Darkness presumably) and looks just like a Darksign floating in the sky. If you do the Usurp the Flame ending, the (still-eclipsed) sun turns white, which seems to suggest that the First Flame really has transformed into something else.
Given that the Usurp the Flame ending is all about your character becoming the Lord of Hollows and will presumably mean the ascent of Londor, I really have to imagine that "The Ringed City" of the DLC will, in fact, be Londor.
I don't know exactly how the theories go regarding Londor. Frankly, I don't recall if Oolacile and New Londo were the same place, but I believe that New Londo might be where Londor exists - so often the locations in Dark Souls are kind of written over the last, like the Painted Worlds and the fact that Irithyl is (spoilers) Anor Londo.
If this is Londor, I wonder what's there. The Dark Souls lore is vague enough that even if it's filled with Sable Church-affiliated beings, it might still make sense for you to fight them even if you're on the path to becoming the Church's prophesied messiah.