I think the two being tacitly forced to practice dancing in McGonagall’s class would mark the transition of Malfoid’s teasing as of Harry from mean spirited to much more fun and purposeful. I say this because I think that in the moment, Malfoid sees Harry looking all improper and disheveled as he steps forward to take her, and an idea comes to her mind.
“What’re you smirking at, Malfoy?” Harry asks in a hiss.
Lyra (Draco) flutters a proud little chuckle. “Nothing. Just that I know you can’t keep up with me.“
Harry tightens his lips, determined now to meet her challenge. McGonagall keeps the music going, and Harry tries to keep up, but Lyra was not joking. She is fluid, effortless in each movement she makes; emblematic of a serpent’s mesmerizing motion. Lyra again flutters out a giggle, and Harry sighs.
“Well you aren’t stepping on my toes, Potter, so you’re mindful at least. You’re forgetting something important, though...”
“What’s that?”
Lyra slides into his personal space as he fails a step, and she simply whispers, without an ounce of tease or hostility, “You don’t compete with your dancing partner, Potter. You trust them to move with you…”
A sway, a swing, Lyra using her small, lithe form to pull Harry into motions he couldn’t possibly have ever done on his own. Balance and fulcrum, into a waltz, into a spin. Everyone is watching them.
“… Just like even now…”
Harry blinks and Lyra pirouettes into his chest, looking up at him with pride in her silver eyes and a slight blush in her pale cheeks.
“… I have to trust in you.”
Everyone claps, whistles, but it’s all mostly at Malfoid as she parts and whips her golden curtain of hair with a proud bow. She rises, smirking over her shoulder at Harry.
“Not everything is a trial, Potter. Sometimes… it’s okay to just practice.”
Crabbe and Goyle are anything but subtle. They hench for Miss Malfoy just like their dads do for Lord Malfoy. They are anything but bright but they still know the moods of their Lady so when she's been extra pouty and sulky they go look for Harry to suggest he cheer her up.
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.