No, JK Rowling won't be making another Harry Potter book, or any one similar to Harry Potter.
When fans demanded more Harry Potter, Rowling did the smart thing, something I will forever respect her for. She said no, that she was done writing for awhile and needed to focus on her family. And that's what she did. She's not like Stephenie Meyer who will do anything her fans say for the money with Twilight. Rowling is independent, she writes from her heart, and she writes incredibly well.
There is "The Tales of Beedle the Bard" she wrote, as a sort of add-on, but that's all. Recently she announced that she's writing a new book, an adult fiction book that is not at all a Harry Potter book. It's adult fiction, so she appears to have moved on. In my opinion, that's for the best. Just like when directors try to make sequels of unbeatable, amazing movies and often times ruin the franchise, Harry Potter is best left alone as it is. It deserves to be.
Cheers!