As soon as I saw the ending to Spidey 2, with Franco making his discovery of the Goblin lair, I was reminded of this article..
http://filmforce.ign.com/articles/359/359431p1.html
It's the IGN review of Episode II: Attack of the Clones, and in delving deeper into it's suckage it went on to elaborate how the wheels started coming off the Star Wars franchise when producer Gary Kurtz left the Lucas camp after The Empire Strikes Back. Kurtz apparently counterbalanced Lucas' more hackish tendencies in the first 2 films.
One quote in particular from the article that immediately shot into my head in relation to the Spider-Man franchise was this:
"Kurtz was adamant the film should not be about attacking another Death Star he didn't like the idea of blatantly repeating such a major conceit within the same trilogy."
Now Return of the Jedi wasn't that bad, but minus the ewoks, the second Death Star, and the all-too-happy ending (the ending pre-Kurtz departure included the death of Han Solo), RoJ could have been far more cinematically significant..a potential pinnacle for the series.
So my knee jerk reaction with regard to Spider-Man 3 is:
"NO SECOND DEATH STAR RAIMI!!! DON'T DO IT!!"
Now with slightly further consideration, I would concede that it would be craptacular to
a) abandon the potential drama of conflict with Harry through his newfound understanding of his father's madness
b) have, as mentioned, a repetition of the "well-to-do scientist befriends Peter but later goes insane through wild experimentation" plotline through a Lizard-as-antagonist story
c) force some kind of revisionist, underdeveloped Venom plot in just because you need him as the villain to cap the series
I think the best option with Harry would be to have him carry the Goblin torch, but not without being conflicted..only through his father's subconscious coaxing. He would work as a dual villain/sidekick, as they could gang up to face the main villain in the end. This of course would require a villain of considerable stature and power, which for me could only be someone like Mysterio or Venom.
I think Venom would be awesome, but I think to use Venom as a one-and-done would be a travesty (a Venom arc could transcend an entire series), expecially considering how much sway the Green Goblin has been given over the initial trilogy. It would also be sad to see them butcher or abridge his backstory just to squeeze him into the film and be done with him. And honestly, if you do Venom in this first trilogy, you leave pretty slim pickings for any future films in the series. Of course in that case you could always come up with film-specific antagonists..remember the sun powered guy from Supes 4?? That turned out great, didn't it??
So basically, I don't know what the fuck they should do with Spidey 3. The safest bet I guess would be a baddie tandem like Lizard/Electro requiring Spidey to make peace with the new Green Goblin in order to defeat them, which would squash the Harry beef as a plot device and ensure no more Franco in any future films *pumps fist*. The only major problem with a plot like that is that super-villain tandems can be a tenuous affair (see the last 3 Batman flicks), but for me it's the only way to get the Goblin in the flick without making him the central baddie again. If Spidey needed the Goblin's help to whup Lizard by himself, that would make him seem like a pussbag. But hey, the good news is, whatever the fuck they choose to do, we'll find out in 2 years, right?