I would change something about Snape's character and arc towards the end of the books. The way they are written, it very much feels like the audience is supposed to end up feeling sorry for him and seeing why he acted the way he did, as though he is truly a hero, however, in my opinion, that is not reflected in his actions at all. He was awful to multiple young kids that he held a position of authority over, particularly Harry and Neville, and this is all supposedly excused by the fact that he switched sides - however, he only switched because Voldemort was going to kill someone he cared about, not because he truly disagreed with the Death Eaters' ideals. Also, the person he apparently cared about so much was someone who was not interested in him and who he had, on at least one occasion but probably multiple, insulted because of her blood status, despite the fact they were supposed to be friends - and none of that makes any sense or makes me feel sorry for him. I would either take out the hero arc almost completely and have Snape being revealed as a double agent be a small detail and not a huge plot point, or rewrite the reasoning behind it and change some of his earlier behaviour to reflect a version of Snape we might actually have sympathy for.
