I agree that Nidhogg is likely a child of Midgardsormr, but the quest line said that two kings among wyrms were present in Eorzea, at the moment. Nidhogg awoke to lead a charge on Ishgard but twenty years ago, and he usually slumbers for centuries at a time. Moreover, the dragon seen in the trailer has both of his eyes. I'm not so sure Nidhogg is leading this new assault just yet.