Oh, a lot. 1. Harry ends up with Luna, not Ginny. With Ginny it’s too much of a cliché (damsel-in-distress ends up with her rescuer) plus Luna is awesome and seems to be the only person who truly understands Harry and is a good match / counterbalance for him. If I could just make one change, it would be this. 2.Neville ends up with Ginny. After being the victim of bullies for years and being clumsy in general, Neville undergoes some big changes in his 5th and 6th year at Hogwarts. He leads the D.A. in Harry’s absence in the 7th year and protects many of the students from being bullied by the Death Eater teachers. This earns him Ginny’s respect and she finally moves on from Harry and ends up happily with Neville. 3.Fewer Horcruxes. Forget about the “magic number 7” (and Harry was an unexpected addition anyway). In the original story, there were too many Horcruxes and the last ones were too easily destroyed. It took six books to destroy one Horcrux (and find the fake locket from Regulus Black) and then all the remaining Horcruxes were destroyed in a couple of chapters in book 7. If there were only 5 Horcruxes (including Harry), the story would be better for it. That is: the diary, the locket, the ring, Nagini and Harry. I would remove Hufflepuff’s Cup and Ravenclaw’s Diadem. It never really made much sense to me anyway that Voldemort would hold those houses in such high esteem. 4. Love potions don’t actually work. Love is a common theme throughout all books and it is often emphasized how Voldemort doesn’t understand love and how love conquers all. Then how is it possible that love potions seem to actually work? Love potions are so powerful and dangerous that they should be in the same category as the Unforgiveable Curses. Even Voldemort was conceived as a result of someone using a love potion. This conception (partly) led to Voldemort being unable to love. While technically you could argue that a love potion doesn’t actually make you love someone and works more like hypnosis, it is still far too powerful. 5.Provide a valid reason for the whole plot of Goblet of Fire and why it was necessary to let Harry win the Triwizard Tournament. For example, perhaps portkeys can only be created with help from the Ministry and this would make the Triwizard cup the only possible option. 6.More examples of good wizards/witches who were in Slytherin. 7.Quidditch. Other people have already covered this. At a minimum: introduce a maximum time limit, the Snitch should only be worth 50 points or so, introduce some kind of safety spell that protects participants who fall off their broom and finally make the playing field much smaller and give all the spectators Omnioculars. 8.Perhaps this will lead to disagreements, but I felt it was a huge coincidence that Harry managed to reach Snape in time for him to transfer his memory for the Pensieve. Instead, I would do the following: Snape already put the memory in a vial and stored it in a place with special meaning for him and Harry (e.g. the Astronomy Tower where Dumbledore died or simply the Headmaster’s Office). That way, if Snape died his portrait could have still told Harry about the vial. When Harry meets the dying Snape in the Shrieking Shack, he just tells him right away where to find it.