4. Emily Blunt (Black Widow)
This one makes natural sense because the role of Black Widow was originally set to go to Emily Blunt anyway, but she had to pull out of production on Iron Man 2 because of scheduling conflicts (Fox held her in a contract, forcing her to star in the terrible Gulliver's Travels instead, though she was the best thing in it...). Instead, Scarlett Johansson nabbed the role, yet frankly (if I can be superficial for a minute), I don't think audiences would've been too cut up if either actress got the role. The role of Black Widow hasn't been a particularly Earth-shattering one to date; the basic requirements are that you must have a quick-wit and be sexy, and Emily Blunt easily fits that criteria. The fact that Marvel Studios had initially landed on her over Johansson says it all, really.