MovieWeb brings us the following word:
There have been numerous complaints from fans that this sequel is just crammed with way too much stuff and far too many villains. We know Electro (Jamie Foxx) is making an appearance, along with Rhino (Paul Giamatti), and Dane DeHaan is starring as Harry Osborn, with Chris Cooper playing Norman. But most people don't think Green Goblin will emerge until The Amazing Spider-Man 3.
Well, we can confirm that Green Goblin will actually be making his big entrance during the climax of The Amazing Spider-Man 2. Here is a photo of the glass case that contains Norman after his transformation, which he breaks out of.