I already posted my half-baked theory in the 7×5 press release thread:
I strongly suspect that Eloise Gardner is the witch whom Victoria (aka Rapunzel) has locked up in her tower. As far as I can tell, she is the only missing person in the story so far…I think it is going to be the old bait-and-switch kind of reveal in which Victoria Belfry turns out to be Rapunzel, who is really a villain, while Gothel is the misunderstood anti-hero. It is a well-worn trope now on OUAT like when Peter Pan was really the villain and Hook was the anti-hero or in which King Arthur was actually a power-crazed schmuck.
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"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy