Gemma Arterton is my favourite for the role.
My vote goes to Eliza Dushku. But for the OP she cant appear in First Class and then Avengers as they are separate companies.
Reading: The Walking Dead, Morning Glories, Saga, Batman, Hawkeye, The Wake, superman unchained, avengers AI, xmen
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Even if they were going to put Scarlet Witch in a film, they aren't going to cast anybody over 29 let alone 30 to play her.
Think Phoebe Tonkin, not Eva Green.