4) Kevin Spacey
Kevin Spacey has already played a superhero nemesis already in Superman Returns – he was Lex Luthor opposite Brandon Routh’s Supes – but, hey, there’s no rule that he can’t play another. He’s generally considered to be one of the best things about that film, after all.
Perhaps the direction of Osborn that would most fit Spacey would be the more recent interpretation of the character in the comics. In the likes of the Thunderbolts and Dark Reign storylines, Osborn does not don a green mask to commit crimes but is presented as a corrupt businessman with dreams of political power.
Not to say that Spacey can only riff off his previous roles, but casting him as this version of the character would be too good an opportunity to miss. It would be a perfect mesh of Lex Luthor and Frank Underwood.