Here's what I think it should be about:
The movie will obviously feature Spider-Man fighting Harry Osborne as the new Green Goblin. But first things first; we start out with, say, a date between Peter Parker and Mary Jane. It’s going smoothly until an inevitable disaster ruins everything. Of course Mary Jane understands why Peter has to leave, but you still see how difficult it will be for them to maintain an intimate relationship. We’d also be introduced to a rival photographer named Eddie Brock *wink, wink* who begins to hate Spider-Man more, and more throughout the movie. Now when the fighting starts, Spider-Man is aided in his battle with the Green Goblin when a meteorite crashes into a building or plain or something, and he is covered with a mysterious black slime while saving people from the burning wreckage *wink, wink*. The mysterious subtance forms a new suit and enhances all his powers, but fuels his anger and agression, causing him to dis everyone he‘s ever loved. He finally realizes he’s got to get rid of it when he finally defeats Harry, but is compelled to kill him for all the trouble he’s caused. He manages to get it off, and put Harry in a mental institution. But elsewhere, the slime has fused with Eddie, giving him all of Spider-Man’s powers… and more. Together, they call themselves Venom, and prepare for the final conflict.
Of course I didn’t put too much detail into this plot. And if they didn’t change from the fight with Harry to the fight with Venom smoothly enough, it would seem too much like two different movies. They’d have to connect the two villains in some way, though I don’t know of any comics that do so. But it’s a start at least. Please submit your own ideas (even if they suck)