Board Thread:General Discussion/@comment-26109288-20150913132446/@comment-24674051-20150914152624

And regarding casting issues, Elsa, in the Season 3 finale, was portrayed by an extra (I don't remember if she was credited or not), and then was cast for season 4 after the fact.

So, particularly with almost 5 years seperation, it would not be out of the question to use one of the other villians from the circle that helped Regina. That would be assuming that they would want to. Most of the villians from that circle were rather non-descript.