First, Mat is NOT DEAD as some have theorized, because, while flying rubble knocked him out, the book left him running toward the sounds of battle against the Seanchan armies, so he isnt dead as dead men dont run. We know that the DoNM is Seanchan royalty, but I doubt she's the empress because we know she already has kids(tFoH in Seanchen, read what the Seeker says) By the reaction of the guy in the Bar in 7, and his thoughts later, the DoNM is related to the Return somehow, but they dont seem to know who she is. The Empress reigns over the Court of Nine Moons, which leads me to belive that the DoNM is perhaps the Empresses daughter, or some other heiress to the throne. As to how Mat will meet her, I suspect he will be captured by the invaders in Ebou Dar, and taken to Seanchan because they'll want to know how his medalion was made so as to make more, or at least where he got it. Hell meet her, and things'll go from there.