Bad idea. I say that although we want more Harry Potter, we don't really because then it would just drag things out. I wouldn't mind an extra (little, plot-less) book to tell what happened to the wizarding world, it's people, cities, stores, creatures, etc. Another series though? She would never write and if someone else was to start it after her death, then they should be banned from the earth. This is her series, not anyone else's, not even her kids.