IMHO, the greatest benefit of the Harry Potter series is that it gets kids to read, and to enjoy reading. They can move on from there, but something has to spark an interest in reading itself. This is especially true in an error when so many alternative options (i.e.,computer games) are availabe.
JK Rowling needs to do one of the following:
1. Write another Harry Potter Book (They need an 8th book to tie up the loose ends).
2. Start a new set of books set in the Wizarding world but with different caracters. Maybe a trilogy?
3. Drop the Wizarding world and do another world—Sci-Fi?
4. Start a new book with new “Normal” people set in England during the Blitz.
5. Go for good detective novels making a new offbeat detective of some sort—be creative!
6. Write short stories for magazines or Dr. Who?
7. Do non-fiction—maybe a biography or autobiography?