I'm sure lots and lots of people adore the idea of J.K. Rowling writing more Harry Potter books. I know I would give in and buy them, as would millions of other folks. The one question I have is, what stories would she tell?
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It's always interesting to see how an author like Rowling, or a creator like George Lucas, handles the expansion of the universe they've created. Rowling initially said she was done after the publication of Harry Potter and the Deathly Hallows, and dropped some initial tidbits about how she understood her universe, the biggest being that she understands Albus Dumbledore to be gay. The plot and time gaps she left open have essentially been filled in by fan-fiction authors (my favorite post-Hogwarts creation is the marvelous After the End, by a couple of pseudonymous authors, which involves Ron working in a bar and later becoming a magical lawyer, Hermione doing advanced magical philosophical study and become a sort of inventor, and Harry being annoyingly self-sacrificing as usual). Lucas, on the other hand, allowed other authors to carry an authorized version of his world forward in books, games, and television shows, while he went back in time to fill in the gaps with more of his own work (and, of course, milked his fandom with re-releases of the original trilogy, and now a 3-D theatrical re-release).
The thing with Star Wars though, was the universe was just so big. Characters like Wedge Antilles who essentially had no backstory or characterization at all in the original trilogy have been expertly fleshed out by other authors, most notably Michael Stackpole. Rowling's universe, by contrast, is much smaller, and any author who followed in her footsteps would have to contend with her extant character sketches.
Most importantly, though, what stories can she tell after the battle with Voldemort? We already know Harry and Ginny and Ron and Hermoine end up happily coupled and with healthy offspring, having essentially ended their feud with an aging Draco Malfoy. What other adventures are there left to tell? The Ministry of Magic was always more of a one-off than a world to be comprehensively explored, unlike the larger problem of how to govern a global republic or federation. A domestic tale of Quidditch and work and courtship wouldn't have the propulsive momentum of Harry's coming final confrontation with Voldemort. Even if she were to go back in time, Rowling already comprehensively—and in some cases, beautifully—narrated the childhood, courtship and adventures of Harry's parents and their friends. Those characterizations, in particular the single, brilliantly executed chapter of Severus Snape's memories, should be sacrosanct. Rowling should let the books go. She told the main story of the universe brilliantly. It's over. And it's fine.