Ok...idk where Electro's been getting this popularity from, I must've missed something, but anyway, here are my comments.
Chameleon: Shouldn't happen. He's not exciting, he doesn't pose a great threat, and he doesn't have any new CGI or anything advanced as far as suspense goes. Go watch T2 if you want to see morphing, its been done, many many times before. Plus, if you say Topher Grace is playing him, that's a stretch, cause actors don't take silent roles unless they need it. Grace is popular enough that he wouldn't be cast in a movie to play a non-speaking role. Church is too old for this too.
Sandman: Good possibility, though he's kinda lame. He has nothin to do with Peter too, so there's a broken link. Possibility of him being cool - I was never a fan though - and a huge chance of him being in there seeing as its the favorite of two of the biggest creative minds on the project.
Venom: Could do him in one movie (I have a formula for it, trust me, it works), great connection with Peter, exciting new villain that brings a lot of money to the theatres and fills seats easily. Also, you wanna top the train in SM2? Here's a thought...Venom vs Spider-Man swinging around the Statue of Liberty. Church doesn't look like him, and neither does Grace, however. Richard C. Everbeck is never mentioned but he was cast earlier in Spider-Man 1...he's unknown and may be lurking.
All in all, we have no 100% factual evidence that its anybody. There are just some we can logically rule out, such as Black Cat, Hobgoblin, Beetle, Kingpin, Smythe, etc etc etc.