I've given this a lot of thought, and I think I know exactly what is going to happen in Book 7....
Ok, so, over the summer Ron, Harry and Hermoine have been out having adventures and during this time Harry and Hermoine get married, Hermoine is pregnant at the beginning of the book and they begin their search for Voldemort. Harry starts seeing dreams where Hermoine dies in childbirth and Harry begins to lose it. Voldemort appears to Harry and tells him he can teach him how to keep Hermoine from dying using the dark side of the magic. Slowly, Harry begins to see things only in a sense of power and the sides of good and evil become shaded. Harry then, under Voldemorts command, takes out the 1st year students at Hogwarts. Ron tells Hermoine what happened and she turns away from Harry, who, in turn finds and tries to kill Ron, and accidently causes the death of Hermoine that he has been seeing in his dreams. He and Ron fight and Harry is beaten and Ron goes into hiding. Harry and Voldemort track down and kill the rest of the wizards. Thus the ministry of magic is destroyed and the new government rules the muggles with fear and power.
The end.