I have the baseless headcanon that Mary Poppins is Maleficent.
Maleficent reforms sometime in the fourteenth century, greatly reduced. She wanders the countryside in the shape of a harridan, another bogeyman to warn children of.
The peasantry - who remember Maleficent as the fey who inexplicably sent her minions to inspect every cradle in the kingdom - think that she is another of these goblins.
They call her La Mauvaise Paysanne.
***
She arrives in England in the eighteenth century on the back of a fairy tale.
Fairies take the shape of the stories told about them. The myth has changed, and so has she. She takes the shape of an enchantress, the punisher of naughty children, the rewarder of the good.
The English people call her the Merry Peasant.
***
By the nineteenth century, the edges of her myth have been sanded off entirely. She’s known as a fairy godmother who blesses good children.
She doesn’t mind this, but when she gets the chance to reshape her wand, she gives it a crow’s head in memory of her old familiar.
They’ve started calling her Mary Pauper.
***
By the twentieth century, Mary Poppins is more powerful than she’s been in centuries.
She’s also changed completely. She understands this to her core, and mostly she’s fine with it.
She misses it every now and then, of course. The palace, the minions, the mistress of all evil. She sniffs and says these are are ridiculous thoughts, but she thinks them nevertheless.
This is partly why she enjoys being a nanny. It’s a good compromise. She can be strict but fair with her charges, and every now and then she meets a particularly incorrigible parent who she can curse into oblivion.
marjorierose