4. Black Cat
With a female-led spin-off set for 2017, the Black Cat seems like a sensible choice for both that and a role in the Sinister Six. We met Felicia briefly in The Amazing Spider-Man 2 where she was played by Felicity Jones, but the sequel missed the opportunity to drop some hints about her being in Oscorp for more than just giving Harry Osborn a helping hand (what if she was there to steal something?). Perhaps this upcoming movie focusing on the villainous team could see her true motives revealed before the Green Goblin recruits her, only for Felicia to see the error of her ways and set off out on her own in time for a movie of her own and an expanded role in The Amazing Spider-Man 3 as a new love interest for Peter Parker. Either way, this is a character with a lot of potential and Sinister Six could be a good place to start.