The Sinister Six are about as coveted a Marvel team-up as you can get outside of the Avengers, and it looks as though they'll be appearing in Insomniac's title as well.
With Electro, Scorpion, Rhino, Vulture and Mister Negative all confirmed (that's five villains, just so you remember), that leaves one last spot to be filled. It looks as though the man in question appears in the demo's closing moments, though who in particular it could be is still anyone's guess.
Most likely, perhaps, is that the man of the hour is none other than Norman Osborn. Insomniac haven't clarified as to whether or not the NYC mayor has donned the identity before, so if he were to appear on his glider - unmasked at that - it's easy to see why Spidey would be shocked.
Others have theorised that the unifying force could be Mysterio, Kraven, or even Venom, but Osborn still makes the most sense. He's the only one with the financial clout to outfit his villainous cohorts in this particular way, and with a mayoral campaign in jeopardy, the timing's perfect for the character to step out of the shadows.