I'd write the romance a little better. In the book, Harry suddenly has a quick fling with Cho Chang in before switching to Ginny Weasly. It seems rather rushed for me so I'd like to give Harry time for his "first love" relationship to develop and even break up. Then he could settle his heart on his best friend's sister. It wouldn't change the entire storyline but the plot development would be different.

One thing I'd change? How about two? The Triwizard tournament is strictly for European schools but I would've liked to see a cameo of Salem Academy where the American wizards go to school..in any of the books.