It would be pretty nice. But I think she already told us enough about Harry before his parents died. Well, things that are really important. But she said that she probably won't write an 8th book, unless it's for a charity.
I think a prequel will be an excellent idea. But JKR kind of puts details from the past into her books. Like Snape's pensieve showing Harry what it was like when James was in school. But I would like to know more about Lily and James lives as Hogwarts students.