As Pabbie explains to Elsa and Anna that the former's powers will have to be enough to save them all, an image from Elsa's childhood flashes by. It's a young Elsa in bed (old enough to be around the time she hurt Anna and refused to leave her room afterward) with her mother at the window, staring out at the Northern Lights.
Bear with us, because this is where the theory gets a little wild: what if Elsa wasn't the only royal to have powers? What one or both of her parents had abilities and that's why they were so concerned when hers seemed so out of control? It seems far-fetched, but there's definitely a connection between Elsa's parents and her abilities! And since the past isn't what it seems like, maybe we'll be learning a lot more about Elsa and Anna's parents this go around.