Board Thread:General Discussion/@comment-4911992-20140522151443/@comment-24674051-20140928121759

Elsa (and Frozen in general) may have been a curve-ball from left field, but I think the larger concept of the snow queen (as in the HCAnderson tale) has been there. There were hints of some malevalent power that deals with ice, going back to the harsh winter when Snow was born, the frozen lake that killed Will Scarlet's sister, and I think there were some other little nods along the way.

Besides, Rumples vault where he keeps dangerous magic was not introduced until the beginning of Snow Drifts, and that is where Elsa was being stored. So the fact that it came from the vault should be a clue that it could be anything. That little (big) vault has become a plot device that can be used to deliver a lot of curve balls (assuming that a way is figured out to get back to the EF once again).