whatever character they choose for the next films would have to have an intricate connection to peter parker. Its how the spidey universe works. While already being used, the green goblin is thoroughly connected to peter parker through his relationship with his best friend harry osbourn. As for non used villains, Electro is just a spoiled brat with anger issues (his powers are cool though), shocker doesnt have much too him to hold a movie alone, while mysterio has the best chance at being in one of the movies, but im just not confident he could hold it on his own.
In my honest opinion, i believe the vulture would be a great second villain for parker to face off with. Vulture has a great connection with parker through his aunt may, and sam was really excited to visit the character until the studio got involved again and basically forced him out. not only does vulture have a connection with aunt may and peter, but also with oscorp which could set the stage for green goblins reappearance in the 3rd.