JKR has said that neither Petunia or Dudley will do magic late in life. That leaves us with very few Muggles left. She has also said that no new characters will be introduced in book 7. So this only leaves Vernon! Is it possible that somewhere in his family there is a witch or wizard that he too is pretending to not have. He did seem to know the Hogwarts logo, without asking Petunia what it was. Could it be Deatheaters will show up and in his effort to protect his family will do magic?