The one changes that needed to be made is to drop Sandman from the movie.
This isn't just what I would have liked to see, but the reason the movie sucked was because there was way too much **** going on and there wasn't enough time to focus on each storyline.
By eliminating Sandman entirely, it would allow more time to develop the symbiote storyline, and really show the effects of the suit, as well as adding much more build-up for the eventual appearance of Venom. At the same time, Harry's subplot as the New Goblin could have greatly been expanded on and would have been soooo much more effective.
But having Sandman in the movie was just too much, and the only reason he was in the movie was to 1) sell more toys and 2) thrown in more action. It would have been better because he was barely in the movie as it was and his origin was so uninteresting. He would pop up for a second, look at his locket with his daughter's picture, and then disappear for a whole chunk of the movie, only to return for the next action scene. With him gone, Harry could have been a much more convincing villain, and Venom could have turned out much better as well.