It can't be MorrowSeer, since it's stated by Mastermind, "No dragonets either? But I have a son!" And he already has a daughter, not known by him, which is Moonwatcher. So who can be Greatnesses father? It has to be a strong, large nightwing. Perhaps Strongwings? That might be unlikely too, since it's never stated Strongwings has a kid or had a kid or never had one- but it's not possible.