Doctor Octopus
Who should play him? If you want loyalty to the comics and something vastly different from Alfred's Molina's iconic turn, getting a nutso European actor to take on the role of Otto Octavius and give him a 'crazy scientist' vibe would be fun. My money would be on someone like Udo Kier or Bruno Ganz. For someone younger, Til Schweiger would do.
Why would/should they be included? As the only major villain that hasn't as yet been alluded to in this Spiderman series, Doctor Octopus seems like a good choice. He originally founded the Sinister Six and was its leader. He's just as famous as Green Goblin, so he'd be a popular choice as well. But it'd take a whole movie to properly introduce such a major character, and TASM3 may be occupied with introducing us to a Goblin or a Vulture, or both, as well as setting up The Sinister Six proper. As well as the fact it'd be classic a case of "too much too soon", there is Alfred Molina to consider. Of course someone will play Doctor Octopus again, but like Tom Hiddleston's Loki or, more importantly, Heath Ledger's Joker, the last guy to play the villain was so perfect aesthetically and in his portrayal, that that really is a tough act to follow, almost impossible, and in a film series significantly weaker than the Raimi Trilogy (even the fans know I'm right), it's not in a good place to set up such a great character.
Likelihood: 2 out of 5 Doc Ock is a possibility considering how vital he is to Spiderman's mythos and to the Sinister Six as its original founder and leader, but with Green Goblin likely to show up and the shadow of Alfred Molina hanging over whoever took on the role and still fresh in the audience's minds, I have a feeling we won't see him for some time, and certainly not as a member of the Sinister Six.