Heartwarming / Harry Potter and the Nightmares of Futures Past
When Neville asks Harry if he's alright after a disastrous first potion lesson when Snape deducts 100 points from Gryffindor, Harry instinctively calls him "Nev", a nickname from Future!Harry's timeline when he assures the boy that he's fine.
Molly and Arthur offer to become Harry's legal guardians after Vernon beats him within an inch of his life. Harry is moved to tears by this.
Harry eventually tells his friends that he's from the future and tearfully confesses that he's guilty of manipulating all of them. Cue group hug.
Then Ginny tops it a chapter or two later with this line:
Ginny: We were more than friends, weren't we? In the future, that is.
Sirius escapes and Harry reunites with him, telling him of the bad future he came from. Sirius remarks about the endeavor being one of the greatest pranks of all time and remarks. "Somewhere your father is grinning like a maniac."
Hermione makes a comment about people only wanting to be friends with her so she'll do their homework. Ron then immediately informs Hermione that he doesn't want/won't take her help with homework, because he wants her to realize that he's her friend for her, not for her homework skills.
After Melissa Bulstrode dies (in apparent revenge for her warning Harry about a couple of things) her sister Millicent starts to become bullied by her Slytherin companions and threatened not to tell anyone about it. Harry, feeling responsible (because he owed a debt to Melissa) finds a way to get her to go to Beauxbatons without calling undue attention, and funds her entire education there.
When Millicent returns during the Goblet arc, she has been taken under Fleur's wing, with the elder girl teaching her confidence and how to carry one's self properly. The main characters even manage to befriend the Beauxbatons students this time around, thanks to Millicent.
Percy loses his job in the Ministry after he attacks Barty Crouch Sr. when he's about to act after his son is revealed. His actions, though, help him land a much better job as Amelia Bones' assistant, since she respects him both for his bravery and for his smarts.