Actually, I'm thinking Venom will be the "half villain", since he'll be Eddie Brock for most of the movie. I think the main villains of the story will be Sandman(a given) and, in a smaller role, GG2. I think Eddie Brock will be presented as more of a nuisance for Peter rather than a villain, but grows steadily more hateful towards him. Only near the end will he become a true villain. And I think they're going to start and finish the Venom business within this film. As others have said, they want to provide closure with this one.
I personally don't think Venom needs an entire movie. The way they're handling him in this one is perfect for his type of character. He's not the kind of villain who can hold an entire movie. In the comics, they've had years and years to flesh him out and show the different sides to him, but with film they don't have that luxury. He's going to come across as little more than a weird alien monster, and it would take an entire franchise of his own before you could start to shake that image. Now that "weird alien monster" type of character makes a fine bad guy for horror movies, but for movies like Spider-Man, the villains have to be more humanistic in order to be interesting for long periods of time.