Ideally, they'd cast an unknown who looks like the author intended. But since they won't do that, I'd cast Emma Corrin, but we'll probably get Millie Bobby Brown.

Author P. L. Travers was very firm about Mary Poppins' appearance in the novel's illustrations, working closely with illustrator Mary Shepard to create an image of the character. Eventually they based Mary Poppins' appearance on that of a Dutch doll: tall and bony, with short black hair, large blue eyes, a snub nose, and a prim, pursed mouth.