The creation of a horcrux requires an evil and violent act, murder being the most violent and unforgiveable act anyone can do. When he tried to kill the boy after murdering his parents, he wasn't able to because of the protection his mother gave him Harry by sacrificing herself. When the killing curse backfired, it left a piece of Voldemort in Harry, which is why Voldemort was weakened to near death.
Like Fury said, he left a piece of himself within Harry, including some of his traits, like being able to speak in Parseltongue, a trait not commonly found outside of Slytherin. Also it could be that Voldemort whet to the house all along to make Harry a Hrocruz, since it is believed that Harry himself is a decendant to Godric Gryffindor, and Voldemort used relics from each of the heads of the houses, ie Slytherin's locket, Hufflepuff's cup, and an unknown object of Ravenclaw's, which leaves Gryffindor's item!
Again just speculation. But it could be the huge shocker of Book 7, that Harry sacrifices himself so that Voldemort can never come back. I think it would be the only way to truly end this series. It leaves Ron and Hermione together as a couple, and the new leaders of Dumbledore's Army.
I think that Severus Snape becomes the new headmaster at Hogwarts, yes he becomes the new headmaster! What he did was to protect Draco, and Dumbledore knew this, he willingly sacrificed himself to protect Draco. Snape in fleeing continued to give hints to Potter on how to defeat Voldemort. I think he helps bring down Voldemort in book 7, and takes on the duties of headmaster!