Well I was thinking Blizzard. Maybe he takes after Aurora more and can change into a human as well? Im think Charcoal and Dusk, while they come to visit Berk every once and awhile, mainly travel, while Blizzard stayed in Berk all the time. But even though he stayed there would not let anyone, not even Hiccup or Castiel ride him, until one day they find their little daughter(maybe 1-3 years old) going around the village on his back(not flying of course, just with him walking) and Blizzard basically becomes her dragon.
Another one of my ideas is maybe Drago had a son who wants revenge on Hiccup and Castiel for killing his father and during an attack(it would be a sudden attack where they wouldn't know Drago's son was the one ordering the attack)their daughter, who is still rather young being 4 or 5 gets taken. Drago's son figures he can raise the daughter as his own(either she would be too young to remember her parents or something would happen to make her loose her memories) and use her to fullfill his revenge and either killing her in front of them or telling her to kill them.
I have a couple of ideas of how they could possibly get reunited but I'll wait to say those once you tell my what you think of all of these ideas so far XD