Chameleon
Who should play him? For a man of many faces, why not get an actor who has built a career out of hiding behind masks? Doug Jones is the obvious choice and looks unnervingly perfect, but my ideal casting for Chameleon would be Julian Bleach, so underrated and gave the greatest-ever portrayal of Davros in Doctor Who. Give this man the star-making role he deserves!
Why would/should they be included? For those unaware of who this dude is, Chameleon is actually the first villain Spiderman ever faced in the comics. He's basically a guy who can change his face to become anyone with the help of various technologies, as well as altering his skin so he can become whoever he wants to be. A facial shapeshifter is a great idea, but is it one that would fit in well with the rest of The Sinister Six? I guess the issue facing Chameleon is that he may appear to be a weird kind of combination of Mysterio (illusion) and Venom (taking on someone else's form), or the Symbiote at least, to an audience. By introducing him you could make a Spiderman film into a political, paranoid thriller, but that doesn't sit well with the style already established. Plus, he's not someone synonymous with The Sinister Six, a little bit of an obscure villain, especially to the public, and he wouldn't stand on his own two feet as a prominent antagonist. He'd, yes, fade into the background.
Likelihood: 1.5 out of 5 Much like Kraven, a fascinating character that would add an interesting dimension and help to ground The Sinister Six, but because he's even more obscure I don't see why anyone, bar me, would be clamouring for his inclusion.