JK Rowling is back in the Potterverse, writing the script for Fantastic Beasts and Where To Find Them, the story of Newt Scamander, who wrote one of the main textbooks used at Hogwarts School For Witchcraft and Wizardry. Set 70 years before the events of the first Harry Potter film, the movie is being described as neither a prequel or a sequel, so I guess it's just a plain old spin-off.
It'll be a spin-off, but not a one-off. A New York Times profile of Warner Bros honcho Kevin Tsujihara reveals the film is seen as the first in a trilogy. That's not wildly surprising - after all, Warner Bros turned one book into two to stretch out the series - but it's interesting that it's being developed up front as three films. Which means JK Rowling is giving them lots of material, as she's totally onboard. The Times quotes her:
When I say he made ‘Fantastic Beasts’ happen, it isn’t P.R.-speak but the literal truth,” Ms. Rowling said in response to emailed questions. “We had one dinner, a follow-up telephone call, and then I got out the rough draft that I’d thought was going to be an interesting bit of memorabilia for my kids and started rewriting!”
While it's not a prequel it seems likely that some of the older characters from the series - like Dumbledore - will show up at least once. But the events of the movie take place before even the reign of Lord Voldemort, so it wil be a very different wizarding world we experience.