Brock's character has to be fleshed out before he becomes Venom. They have to make you believe Brock's hatred for Peter and why he wants him dead. Green Goblin will come, but Venom definitely needs to show up. He's Spider-Man's ultimate villain.
Put the black costume in Amazing Spider-Man 3, use the crashed space shuttle for its origin. Introduce Jensen Ackles as Brock, the climax will have Spider-Man lose the symbiote in the tower, then post-credits we see Brock as Venom.