Just saw SM2. My eyes were glued to the screen the whole time. I feel that they set it up so well for SM3 but they messed it up. I think that there would've been a solution for SM3 if they: Had Harry struggle with the Goblin formula and just wear a darker, hooded version of his father's suit WITH the Glider and Bernard passes away so he can never find out(But although he lives a lie at the end he has a confrontation with Peter and finds out about his father but is so crazed that he doesn't believe him and ends up dying a la SM 1 or some other way and apologizes for his actions, No Sandman, a more fleshed out Symbiote ark and a better MJ/Peter story in which they find out that being together is harder than they thought. Also, yeah, make it a two-parter with Eddie Brock at the end becoming Venom. I think Harry being the main villain would work.
This is more in line with what I always had in mind for SM3.
For Harry, I always felt like they needed to spend a movie on him dealing with the revelation of his father being the Goblin. At the end of SM2, he almost seems to go crazy a bit, which could've set up for him deciding to isolate himself which he fought the demons of his father as he questioned who he loved more, his best friend or his father. Since they never showed whether he took the serum at the end of SM2, I always thought they could have played with that a little more.
As for Venom, I stand by my belief that everything done with Eddie in SM3 was great, with the exception of actually showing Venom. I think if the film ended like the 90's cartoon did, with Eddie being consumed by the symbiote in the final moments and a slight reveal, it could've been perfect. The symbiote could've also been a villain, possibly leading Peter to take MJ to the church because it had messed with his mind, and as he was about to attack her, she knocks the bell, causing the suit to react. In a way, it would seem like she was the one saving him, which would've been a nice change of pace. After the symbiote finally comes off, they could swing away together, and Eddie getting it could've been the final scene or an after credit scene.
Sandman could've been kept in as the main villain for SM3, but he need to be fleshed out more. He didn't need a ton of screentime, since Ock was missing from a huge chunk of SM2 and still turned out to be a great villian. But more was definitely needed. And they needed to cut the Uncle Ben/forgiveness angle.