"In the end, just as Voldemort was killed for the first time by a love he could never understand, he is slain for the final time by a love he could never extinguish."
The most perfect layout for the story would be:
-Initially rebuffed by harry on the train in book 1, initially she's starstruck and wants harry as a lackey out of vanity
-Over the course of the next several books, she grows a deeper appreciation for him
-Chamber of secrets seeds her admiration, she duels Potter and despite all her snide remarks, she gets humiliated by him. Initially she's upset, but there's a small short scene of her feeling a bit proud that Harry is so skilled.
-Her feelings transform from respect and admiration to a HARD crush during the Yule ball, both of them get left behind by their dates, and after initial snide remarks and jabs, they dance together, just a single short dance. However, it's only later that night that she realizes why her heart flutters when she sees his face.
-Half-Blood prince is when Harry begins to fall for her, during his observations of her movements with the Marauder's Map. What initially is him cynically observing someone he sees as an enemy becomes concern as he realizes how over the last year she's become a wreck. No longer the stunning and confident girl she used to be, she's tired, depressed, and Harry tries to help. It's that exact concern that blinds him to the fact that she's become a Death Eater, and it dooms Dumbledore.
-The bathroom scene is where it becomes tragic. Harry sees her as what she has truly become but been suppressing when she's at Hogwarts: A death eater. Forced to work for the man who killed HIS FAMILY. Threatened with her life and the life of her own parents if she doesn't kill his own beloved mentor. She loses the fight, she nearly dies, and the whole time afterwards she wishes she did because then at least she would get away from what she has become.
-The biggest deviation from the original story would be that after the rescue in the room of requirement, she doesn't disappear from the story. Instead, Voldemort keeps her around with the desire to groom her into part of the next generation of Death Eater, as his right hand. He forces her to do terrible things, and all the while she hates herself for being too weak, too cowardly to fight back. However, it's only when Harry arrives during the seige of Hogwarts that she recognizes that she has to make a choice. She witnesses Voldemort laying into Harry, winning the duel, and chooses to break the Death Eater ranks and turn her wand on her master.
-She confesses her love for Harry, and he reciprocates those same feelings for her. They both carry baggage, a pile of mistakes and unresolved issues, but they understand and care about each other in a way nobody can possibly hope to understand, they are opposite sides of the same coin. Their pure, authentic, mutual love burns Voldemort like nothing else, a familiar pain that terrifies him.
In the end, just as Voldemort was killed for the first time by a love he could never understand, he is slain for the final time by a love he could never extinguish.