Mysterio
Who should play him? Fans are forever suggesting Bruce Campbell, but I imagine that's only because of the Sam Raimi connection. Since Mysterio should be someone with a sense of theatricality and arrogance, as well as subtlety and complexity, my choice would be Sir Kenneth Branagh. Mysterio always works best behind the dome, so it should be all about the voice, and a stuntman or someone more physical can easily don the costume without ever showing their face.
Why would/should they be included? Mysterio is a cool character- I have fond memories battling him on the PC version of Spiderman 2- but both too bgi and too ridiculous to fit inot the current Marc Webb films. Like many of the villains on this list who haven't already been introduced, he's too major to just include in the lineup now. And if he was, he'd better be a layered and engaging character which Mysterio just isn't. In a Psychotropic Spiderman movie where reality is questioned, he'd be perfect. He was the one Spiderman villain that Sam Raimi should've tackled. He'll show up at some point, but it won't be as a a member of The Sinister Six, I guarantee you.
Likelihood: 1 out of 5 Mysterio was a character I could've imagined fitting in well to Raimi's Spiderman universe, but I don't see him turning up here. Too major to just chuck in at the last moment, but unlike Kraven he is an original member than can't just turn up in proceedings without some foreshadowing, and it seems a little late for that. Plus, what can someone like that bring to the table as an engaging character? I think even the current creative team would agree with that.