I believe that Rebecca will play the Wicked Witch of the East thought to be killed when Dorothy dropped her house on her. Now I do think that she is related to Regina as being either her aunt, or her baby sister. But I’m really going with it being Regina’s baby sister who had been sent away to Oz by an enraged Rumpelstilskin due to the fact that Cora cheated him out of Regina.
The one issue I have so far is that Cora is such a power hungry crazy person that I can’t see her NOT going to get that baby back, not out of maternal love but in order to best Rumple. Also, Rumple may be very angry/hurt by Cora and her actions but I don’t know that he’d outright hurt her infant. We don’t know why he was holding Regina but I don’t think it was to spirit away. After all, she was supposed to be his child. All the times Rumple had to hurt (actually hurt) Regina and he never took it. I think some part of him saw that she was Cora’s and therefore never laid a finger on her. Rumple said in Welcome to SB “Cora and I may have had our differences but she’ll always have a place in my heart.” So I don’t think he’d whisk her baby away after their THING ended.
As to the Witch of the East. I am going to point out that while we are crazy people who study these stories to death, most of the audience, about 9-11 million of us, don’t. The internet fan base is a nice portion but not as singificant. So having it be any other witch from OZ EXCEPT the Wicked Witch of the WEST would likely confuse the audience because for the most part their sole association with OZ is the 1938(9?) movie in which the only Witch you really meet is The Wicked Witch of the West. They have to try and keep things somewhat familar. So, yes Peter is a devil child, but he as certain symbols that signify him AS Peter Pan: the green clothing, the pixie dust, the constant game playing, the never wanting to grow up, thinking lovely thoughts. I’m not sure your casual audience member would be able to associate anything with any other witch from Oz except the Witch of the West.