This is just speculation, but I wanted to get some extra opinions.
After seeing the movie last night, I'm pretty sure one of the remaining villains of the Sinister Six will be the Chameleon. I'm also thinking that he was already in the movie as Dr. Menken, played by Colm Feore. Being a villain with no supernatural powers, it would make sense he'd want to be the brains of the operation, leading Oscorp and the Sinister Six program. Had it not been for Harry taking matters into his own hands, Menken probably would have succeeded. He was also Norman's right hand man before he died so he must have had a lot of influence as well as access to everything Oscorp related and its future.
To be honest, had I not played The Amazing Spiderman 2 video game, I wouldn't have come up with this idea. I'm not saying the game follows the story of the movie considering most movie games don't, but this just stuck out to me. The other major factor here is that just this morning I saw the picture above, and I doubt that will be Mysterio. My line up would be the Green Goblin, Rhino, Doctor Octopus, Vulture, Chameleon, and Kraven. I am still curious to see where the Black Cat will end up in this franchise now that Gwen is gone and Mary Jane may not be seen at all.
As far as the movie goes, I'd give it a four out of five. The villains were underutilized but it felt much more like a Spiderman movie than the last one, and had a lot more heart. This is easily the best superhero film I've seen since The Wolverine last summer, but that's just my opinion. With all that being said, Spiderman is and always will be the shit, and god bless to everyone!
Picture source: http://www.iamrogue.com/news/movie-news/item/11178-the-amazing-spider-man-2-credits-reveal-the-line-up-of-sinister-six.html