Movie games are usually bad, but why? They try to stick to just what the movie had and the developers take comfort in the fact that people will buy them just because it has the movie stamped on it.

If developers were more careful to create a quality product, as careful as when they make say a Resident Evil or Final Fantasy game, then the quality would shine through.

For example, if Harry Potter created a dueling system where you could train and improve over time and they opened more spells and they even allowed you to go bad (play as Tom Ridle for the second part for example) and added so many more layers to the depth. If they worked on graphics for Harry Potter games like they do in Assasin's Creed and made the game have many choices like Mass Effect, they could be good. So the player could ELECT to follow the movie, and get the movie ending. Or make different choices and get different endings.

Sure it would deviate from the movie a lot, but it's not the plot that matters, it's the universe that gamers want to spend time in.