-The “wyvern/dragon lord” is clearly Edelgard (she has axes plus the empire seems like the place with the dragon knights; the “holy kingdom” sounds like the place with the Pegasi). Her special class will be like the Revenant Knights from Fates (axes + magic).
-Byleth cannot romance any of the students save Edelgard, Dmitri, and Claude (the students can romance each other, of course). Anyone else except Geralt is fair game.
-Possibly wishful thinking, but Dmitri’s “tinge of darkness” is just that he’s gay (why else would the Lions be the only house with a gender imbalance?)
-Ingrid is Dmitri’s (possibly twin) sister (same hair color, same eye color, same collar that no one else has)
EDIT
Actually, Ingrid and Dmitri don’t have the same eye color, but they still look too similar for it to be a coincidence.