JK Rowling was recently at the Bath Literature festival and got a chance to speak about 'The Casual Vacancy' and 'Harry Potter.' When asked whether she would ever return to the Harry Potter series, she didn't say no, but she did say that she wouldn't write about the Marauders because she didn't find prequels to be good, in general.
This would mean that although stories about Lily, James, Sirius, and Lupin growing up are ruled out, stories about Harry, Hermione, and Ron's grandchildren would not be.
She also spoke about other aspects of the Harry Potter series, and said that her proudest moment related to the books came when she finally admitted that Dumbledore was gay, and this led to a fan coming up to her afterwards and admitting that he was also gay. In relation to Dumbledore, she reveals a tiny tidbit about the story [which most fans have already guessed], saying that Dumbledore's love for Grindelwald was unrequited and Grindelwald was just using him to get to the Deathly Hallows.
Finally, she reveals that she's already hard at work on her next book, which will be aimed at 8-year-olds. This means we still won't be getting a Harry Potter sequel for some time, but at least she's been thinking about the possibility.