Here's my prediction. Bury it in the time capsule. Dig it up in 20 years when it's all come to pass.
Yes, Ash is right, she'll take a year or two off, spend some time with the kids and husband. Then she'll get to work on her first post Harry Potter book. It'll be something completely unrelated to Harry Potter. It might come out at about the time of the last HP movie, but that, I think might be merely a coincidence. It won't do as much business as Harry Potter by half. I suspect that a large portion of Harry Potter readers are like me, people who have basically lost interest in the story, but continue on out of curiosity or because we've already come this far might as well see it to the end. The (relatively) low sales figures will prompt the press to start calling her a has been and a one hit wonder. Not wanting to tarnish her reputation and not needing the money she won't write anymore. Until... well of course the offer is already on the table for her to come up with a new Harry Potter story. That will never go away. As time passes though her publishers will sweeten the pot until the paycheck is too much for even the fabulously wealthy J.K. Rowling to ignore. I suspect the next book will tell the story of the death of Harry's parents. It's a story that HP fans would love to read. It's got many of the characters you love; Snape, Hagrid, Dumbledore, Voldemort, Jar Jar Binks. And or course it will be a smash hit best seller.