There's something in the games 'lore' about Halfkins having a the full beastkin gene that can come out. So any halfkin mother could have a beastkin child... with any race as the father. I'm not sure it functions this way in game. But it would be interesting to have a new halfkin race, which had a powerful beastkin variant that could only be obtained through breeding.
Or hell, just do it with dragonkin. Give them a chance to give birth to full on dragons!