DON'T READ THIS IF YOU HAVEN'T FINISHED BOOK SEVEN!
I'm watching clips of an interview JK Rowling gave where someone asked her who, 19 years down the road, the headmaster of Hogwarts is. She said it wouldn't be McGonagal because she's getting on in age so it would have to be someone new.
Personally, I think George Weasley would make a perfect headmaster. Just think about it. Jo, you've got my number. Call me.