I solemnly swear I am up to no good... (err sorry, got a bit carried away). I solemnly swear that this is my own theory and I have not nicked it from anywhere. Any similarity to a similar theory or the actual book (going to be released in 3 months from now) is purely coincidental. If I had the literary genius of Rowling-di, I would have written something up myself. Here I present just the threadbare theory.
I would like to clarify at the outset that this theory springs from my love of Severus Snape. Yes, you heard it right.. I love snape! May be it is just Alan Rickman playing brilliantly. To be serious, I can not forget that snape saved harry's life in philospher's stone; I can not forget how snape shielded harry, hermione, ron from sirius in PoA when he thought sirius is going to attack these three (rewind and see the movie sequence inside shrieking shack); I can not forget what a brilliant student prince had been (would have loved to be a student like him) and above all DUMBLEDORE TRUSTS SEVERUS. So the hero of this theory is Snape. Enough of prologue now the theory...
Dumbledore, Voldemort and Snape are the 3 leading wizards of our time. Harry is fine but will take time to mature. It is not possible to say who among these 3 is the best. One thing is sure, Snape is out of limelight. Dumbledore wants to work this to Order's advantage. Dumbledore knows he can not finish voldemort alone. Harry will need to finish voldemort all right. However dumbledore needs trusted and able wizards to help Harry like he himself did. The obvious choice is Snape. Snape is cosy with voldy's group too. So Dumbledore decides to sacrifice his life (which was anyway nearing its end) in the hand of Severus Snape. That explains the pleadings to Snape on top of astronomy tower. He was pleading Snape not to spare him, but kill him. He had also asked snape to create a horcrux for alternate-Snape with the murder. Snape first refused to do this murder and horcruxing business. Then Dumbledore had to convince/argue with him in the dark forest, reminding Snape that this murder he has agreed to do when he joined the order or something like that. Hagrid overheard part of this conversation and Harry suited this conversation to fit his own theory. Snape kills his own headmaster, creates a horcrux and stashes it some place safe. The other snape will join voldemort's group. When harry comes to kill voldemort, snape will help harry, even at the cost of his own life. Harry succeeds in finishing voldemort (or for the effect he may forgive voldy, that is not crucial). All is fine now. Harry will finally understand what snape has done and go recreate Snape from the horcrux. Recreated Snape takes over as headmaster (may be after McGonagall).. it will be a shame not to let such a genius be the principal. Harry will be allowed to become the DADA teacher (this one is just for fluorish, not necessary to the real plot).
That's all about it. ting ding. May I get the literature nobel now?