JKR didn't fully flesh out her universe at the start. She had feature creep just like any app, she added tons of spells and stories and characters and enriched

JKR didn't fully flesh out her universe at the start. She had feature creep just like any app, she added tons of spells and stories and characters and enriched the world book by book. While HP:DH does have a "summary" vibe, going back to all the old places and seeing all the old characters' plots wrapped up nicely, I do think she needed SOME new material in there. She needed to create new tension, she needed new suspense; simply wrapping up things she'd already devised wouldn't have been fresh enough for her, or for us, I think. If I had to change just two things about DH, I think I'd remove the epilogue (it could just be an online essay, there's no need to put a second bow on top of the present, you know?) and I'd remove wand lore. The idea that your wand changes allegiance whenever someone bests you in any way, physical or magical? Come on, that is borderline plot manipulation. Now, Harry wins the fight at the end because Voldemort's wand knew that Harry's hands pulled some wands out of Draco's hand, who in turn disarmed a Dumbledore who wasn't trying to not be disarmed? I mean, ugh, wand lore was a sort of hand-wavey MEH aspect, IMO. I think Book 6 and Book 3 are the pinnacles...they don't feature battles with Lord Voldemort, they're nicely self-contained, they have fantastic endings, I'm getting chills thinking of them.