Seven years not just for Hogwarts, but also because it's often a special number in magic lore. Also, the main story has been dealt with, that between Voldemort and Harry. I'll admit that there's alot more that could be done, but she seemed to have a very concrete idea of what the books would be about when she started, and that's been reached.

She's gotten rid of the main story-line that created any tension/threat/problem. Unless the other books would be pure comedies or less interesting dramas (cleaning up the Ministry could be a story, but who really cares about that?) there's nothing to do except prequels. On that note, though, the prequels could be pretty good, but there wouldn't be as much suspense, since we all now know how things turn out.