Electro
Who should play him? Jamie Foxx, for the same exact reasons as Rhys Ifans and The Lizard. He's an even more recent addition and I get the feeling he's going to be fantastic as Max Dillon and a creepy-looking Electro. The audience will likely love him, so he may even be set up to become leader of this Sinister Six, so to recast would be insane. But assuming he's signed a multi-film contract and has been given a ton of cash, and he's doing this for his kids, it's not likely he'd turn down a reprisal of the role.
Why would/should they be included? If plans are in place to work up towards The Sinister Six, I'm guessing TASM2 foreshadows this in a big way. Starting with its villain, already part of Oscorp, as was Lizard, as are the Osborn family. Electro should be included in the lineup because it makes sense, and to be honest it's the one thing I'm actually interested in about this movie. Jamie Foxx seems to be having a ball playing the bad guy, and he's looks awesome! I think he'll end up being the talking point of the movie, so once you have a popular villain and there's room for him to return, why wouldn't he be included?
Likelihood: 5 out of 5 Out of every villain on this list, I'd say Electro is the only surefire guarantee. But it all depends on where he is by the end of TASM2- maybe he even snuffs it, too early to tell! But somehow I doubt it if only because they'd be mad to lose an enthusiastic, big-name star!