Somewhat related, in some folklores, fairies are known for abducting small human children, in some cases swapping them with their own offspring (called changelings), and I like to imagine what could be their motivation and what will ultimately happen to the kids.
Like do they kidnap out of pure malice or as a cruel prank, are they attracted to the child's innocence, is that how they bolster their numbers or do they simply want to play eternally in Fairyland? How is Fairyland like anyway, an idyllic landscape or a hell on earth where the infants are slaved away until they expire?
And what about the changeling kid, how would the human parents react? Raise them with love, with disdain or straight up abandon them out of contempt? And once they grow up, will they take after their foster parents or will their fairy nature prevail?
I feel like folk tales could still provide some interesting stories even if they'd be quite different from your usual modern monster girl media and veer more towards traditional fantasy.