Over on the latest IGN Movies Keepin' it Reel podcast (I'd link to it, but can't access IGN from work), one of the guys - Stax, I think - suggests something novel: that ASM2 begin mid-battle with a baddie only featured in that one opening sequence, much like a Bond opening. He makes the rather interesting point that, despite these superheroes having extensive rogues galleries, pretty much all such villains figure into the main stories of the movies at hand. The closest exception I can think of offhand is the Sentinels in X3, and those of course aren't actually real.
It's something I've thought about as well. I like it because it's a way to include some minor villains who you might not ordinarily get to see.
The official synopsis has been revealed:
In The Amazing Spider-Man 2, for Peter Parker (Andrew Garfield), life is busy - between taking out the bad guys as Spider-Man and spending time with the person he loves, Gwen (Emma Stone), high school graduation can't come quickly enough. Peter hasn't forgotten about the promise he made to Gwen's father to protect her by staying away - but that's a promise he just can't keep. Things will change for Peter when a new villain, Electro (Jamie Foxx), emerges, an old friend, Harry Osborn (Dane DeHaan), returns, and Peter uncovers new clues about his past.
Pretty vague really. No detonation.