this question is more or less limited to the books, because most of the changes we would make to the films are probably going to be to make them match the books more closely.
some of mine:
-i wish harry and ginny’s relationship had been set up a little more. thematically, i understand why harry had to end up with ginny, but really, he takes zero notice of her until book 6 when he’s all of a sudden all in love with her. she was into him the whole time, and he knew it, but we never see even a hint of him returning the interest until half blood prince and now she’s his one and only. it happens a little too abruptly for my taste.
-i wish luna had ended up with either neville or dean thomas rather than some random dude that rowling made up after the books were finished.
-the character of cormac mclaggen always bothered me because he just shows up in book 6 and we’ve never heard of this guy before. there’s a couple of other characters like this (romhilda vane comes to mind) but mclaggen is a fellow gryffindor and is the same year as harry and co, so we definitely should have heard of him. even blaise zabini, a slytherin character who’s somewhat prominent in this book, gets name checked at the sorting way back in book 1.
-in book 7, during the “seven potters” escape sequence, it makes absolutely zero sense for them to send the real harry with hagrid, or really even to have hagrid involved in the whole thing. i understand that they didn’t put him with mad-eye or kingsley because the death eaters would have expected him to be with the most skilled aurors, but it would have made more sense to pair him with someone less obvious like tonks or lupin or even bill. i mean, they did want harry to survive, right??
-i’ll just mention one movie thing that would have been such an easy fix. hermione’s entrance at the yule ball in goblet of fire has always bugged me. she looks nice, but she doesn’t look different enough. emma watson is a lot prettier than hermione is meant to be, and that’s fine, but in the book when she’s all done up harry and ron don’t even recognize her at first. they should have gone by the book for this one- straightened her hair, slinky blue dress, etc. that scene would’ve worked a lot better.
-kind of off the subject, but does anyone else think draco had a thing for hermione?