The actor that they got to play Dr. Curt Connors is pretty good, so that may be a sign of things to come.
Anyway, if we ARE talking about six movies, I'd personally like to see this.
Spider-Man 3: Harry becomes the Green Goblin, and he and Spider-Man go at it. A little bit more with John Jameson in NASA. Set up Brock's dislike for Spider-Man.
Spider-Man 4: John Jameson brings back the symbiote from space, and Spider-Man gets it. Spider-Man then asks Dr. Curt Connors to help him with it. But unfortunately, Connors then becomes the Lizard and Spider-Man has to hunt him down, during the course of which he inadvertantly ruins Eddie Brock (Or purposefully. The symbiote IS exerting some control, after all). Spider-Man saves Dr. Connors, who then tells him about the symbiote. Spider-Man gets rid of it and Brock gets it.
Spider-Man 5: Spider-Man versus Venom. Spider-Man can't do it alone, though, so bring in another super-hero. Or at the very least have him beaten through some elaborate trap with the help of Dr. Curt Connors or someone else. At the end of which, they end up stranding Venom on an island, somewhere.
Spider-Man 6: Kraven the Hunter. When the series ends, this is who it should end with. Not Venom, not Carnage, but Kraven. They ought to do the story similar to the way it was done in the comics. Kraven defeats Spider-Man and buries him alive. Taking his costume, he then goes to where they left Venom in the previous movie and fights him, defeating him, which Spider-Man couldn't do, alone. Spider-Man eventually digs himself out of his grave and finds Kraven, but Kraven won't fight. Instead, he cuts Venom loose on the streets. Spider-Man has to chase him down to stop him, and Kraven, knowing that he's beaten Spider-Man, kills himself.