The Green Goblin
Who should play him? Because he's been cast as Norman Osborn, Chris Cooper seems the obvious choice to play Gobby. But then again, Dane DeHaan has been cast as Harry Osborn and Norman Osborn is a dying man. Could this series go the way of Raimi's trilogy whilst skipping norman donning the glider? And if it's neither of the Osborn boys, perhaps Irrfan Khan, who was rumoured as Proto-Goblin way back in 2011? If it's neither of these three, then I'm stumped- Christoph Waltz?
Why would/should they be included? Because he is The Joker to Spiderman's Batman. Unlike the idea of rebooting the Spiderman films, reinventing Norman Osborn and The Green Goblin makes sense since it's been almost a decade since we last saw him onscreen. Like The Joker, Green Goblin is a character open to all sorts of reinterpretations. Plus, the fact Gwen Stacy even exists in this canon means history will be made with Double-G. Green Goblin appearing is inevitable, and it's for that reason his will and should be included in a Sinister Six lineup, despite the fact he initially turned down an offer of membership because he wanted to take out Spiderman alone- how insane and badass is that?
Likelihood: 4 out of 5 We have a bunch of potential candidates to take up The Goblin's mantle, so regardless of who it is, I think it's safe to say we'll be getting some sort of Green Goblin. And with a major villain like that on the scene, it's impossible that he wouldn't be included in The Sinister Six, especially if an Osborn takes on the role and is connected through Oscorp to Electro and Lizard.