Harry will wake up and realize it was all a dream.
Having read the whole book online

I can say you're almost there: Harry wakes up and realizes its all a dream, he's still 10 years old and living under the stairs, and a letter arrives from Hogwarts, except when he opens it, the letter is blank. This is followed by 7 blank pages in the book, and at the bottom of the last is written "The End."
Or was it, the letter arrives from Hogwarts, and it turns out Hogwarts is really a correctional institution for the mentally deranged and Harry's being sent there by the Dursley's?
Actually, though, a few more predictions did come to mind this week:
* Is everyone here simply assuming RAB is Sirius' brother?
* The "gleam of triumph" in Dumbledore's eyes at the end of Book 4, when he learns Voldemort used Harry's blood in his resurrection: My theory is that this is what actually fulfilled the prophecy in book 5 ("Voldemort will mark him as an equal...") and as such, Harry will either be able to successfully avoid being killed by Voldemort with the killing curse, or, the curse will again rebound on Voldemort himself.
* Dumbledore's portrait will be of assistance to Harry somehow.
* Also, JKR said back in 1999 that Harry won't be the teacher at Hogwarts (I forgot this), so I retract this part of my earlier prediction about who will be the teacher - leaving Luna or Neville as the main contenders IMHO.