We should be wary, given how Eddie and Adam are about giving fans the slip. Based on the photo here are a couple of plausible theories.
1) Cora gives birth to the future Witch, a child born perhaps illegitimately prior to her marriage and dealings with Rumple. If this is the case then Rumple’s farsight into the future failed him big time.
2) The child is a legitimate child born in secret to protect her from Rumple
3) Some curveball that we’ll ring Eddie and Adam’s necks for when we see the episode.
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Custodian of Graham's darts, Rumple's spindle and Robin's quiver