JK Rowling (JKR) did sub-creation splendidly in the Harry Potter series.
She had seven books, with increasing editorial freedom in each one, and told the story she wanted to tell. She could have said more than she did, but chose not to do so. Dumbledore has two siblings and is not gay or heterosexual in the books. Nagini? A snake. Rowling wrote her books, gave us a cosmos, and now that completed work is ours to enjoy.
We can write all the fan-fiction we wish. The reader reacts to the seven books of the series and his or her reaction is not authoritative, might even be as interesting, or even better, than the originals! We can all go to Hogwarts in our imagination and tell stories.
JKR in her interaction with fellow readers and in her love of her own original vision has become a writer of fan fiction.
The films are one expression of what the original vision, a literary one, might be. They stand on their own as a different art form, but one subsidiary to the books. Just as Star Wars is fundamentally a cinematic universe, so Harry Potter is a literary one. It all began with the books. JKR gave us the books and we should be thankful.
It is interesting, and fodder for doctoral dissertations, to find out what she was thinking when she was writing. Authorial intent counts, but so does authorial exclusion, inclusion, and self-editing. She could have, maybe should have (in a few cases), said more than she said, but she did not. She gave us a completed vision.
From that point on, her opinions about that completed work might be of historical interest (particularly if she can give us contemporaneous notes that confirm her revelations) for scholars of the seven books. For the rest of us, she is just another fan, though one we might have hopes might even surpass the original works.
JKR gave us something excellent, so unlike most new material, we have reason to hope that she will give us something excellent again. This will not change the seven original novels as much as provide a second and larger thing: the seven subsumed to the larger and better world.
So far we are getting more detail, but not a larger world. In fact, like another great world creator, JKR is making her world more detailed, but is not telling us a better story. One can still safely read Potter and ignore everything else she has said or done without fear of missing anything nearly as good.
JKR keeps coming back to Potter, just as Tolkien never left Middle Earth. JRR Tolkien never stopped tweaking his world, understanding his stories differently (Is Galadriel a Mary figure or not?), and working on details. You cannot fit all his ideas into Lord of the Rings, because there are too many of them and they do not all agree. Like Tolkien, Rowling cannot stop tweaking her universe, answering questions, or continuing being creative.
The result might have been even better stories, though it has not. Just as Tolkien’s notes, even in his son’s brilliant editorial hands, are disappointing if one is looking for more Lord of the Rings, so the dodgy play she sort of wrote, the endless ret-con, the details that JKR adds to Fantastic Beasts are not that interesting. It is fan-fiction by the original author.
That does not mean we cannot enjoy the mildly entertaining Fantastic Beasts film series. As Hollywood entertainment goes, this is not Black Panther, but it is not Fantastic Four either. Rowling fan fiction turns out to be not as good as we hoped, but still amusing.
The books as central canon, imaginative touchstones, continue.
Those of us who loved (or at least really liked) the original books might be disappointed that there is not something better or more in store, but Tolkien taught us our lesson and JKR confirms it. Great world builders create their masterwork, give it to us, and keep puttering away on the margins.
We can and should ignore the puttering and pandering. Meanwhile, JKR is amazingly charitable, gave us a wonderful world, some first-rate theme park attractions, and middling blockbuster films. Most of us should wish to have done so much so well . . . And if she is incapable of equaling Potter, then it is not as if any of us have either.
We can all hope she goes on creating and we can keep hoping.
Meanwhile, the original canon endures as written and as given to the ages.