Sandman
Who should play him? Thomas Haden Church did a decent enough job with a tricky character, but he's part of a different series, so we're not going to see him again. John C. McGinley would be perfect- underrated and more than capable of injecting Flint Marko with humanity, as well as any emotional heavylifting that comes with the part. And he kind of looks like the Sandman in the comics... plus, you know, I'd cast John C. McGinley in pretty much anything.
Why would/should they be included? I guess the concept of Sandman is cool, and might have been even better had Venom not been shoehorned into Spiderman 3 against Sam Raimi's will. But he's a character that needs a lot of time to develop, and he just wouldn't be given any justice in amongst a developed set of story arcs an the build-up to Sinister Six. Sandman could've been included in Flint Marko was alluded to or referenced- and he may well be in the new film- but like Alfred Molina and Doctor Octopus, he's been on our screens too recently, and unlike Molina's portrayal, he isn't fondly remembered. He may show up, but it shouldn't be any time soon, and not before The Sinister Six show up.
Likelihood: 1 out of 5 Though he may have been part of the original lineup, I think the fact he's already shown up in Spiderman 3 has ruined his chances. It's not like he's Venom or Green Goblin, he's a character that's not particular popular with the general public and doesn't seem to fit ino the current canon in any suitable way.