I'm betting our friend Ferrovax will be showing up quite a bit. As he could drop Harry with but a fraction of one word of his True Name, he's quite powerful.
Plus...
SpoilerAccording to the Wikis, Butcher has said that Ferrovax is not just any old dragon, but more like Nidhogg, gnawing on the roots of the World Tree. Just as well, it mentions that in a three way fight between the Leansidhe, the Eldest Gruff, and Ferrovax, that the latter would "curbstomp" the other two...