Granny would become a new GHW in the future
Not immediately, but after they return to Norway and conquer the castle. Then, she would lead the two species to co-exist.
I know that in the end the unnamed protagonist (this is for the book, I've never seen the movie) is still a mouse, but this is wild. Perhaps becoming a human again made him a crazy old man. This could explain his ability to communicate with the mice. And of course, his childhood dream was to have his very own Mouse Circus. Also, if you merged the universes, the Other Mother could totally be a Real Witch