4. Sienna Miller
Sienna Miller is another highly versatile actress, this time helped by the fact that she is English and therefore seemingly much more likely to secure the role. Miller has impressed audiences with powerful, damaged, turns in films including Layer Cake and Factory Girl, whilst being unafraid to stretch her action skills in efforts such as G.I Joe: The Rise of Cobra. A beautiful and talented actress, Miller oozes sophistication outside of her films, a trait she definitely shares with Kate Middleton, and is also the same age as her, which although not exactly a necessity, would go some way to helping her bid for the part. On top of that, Miller's glamorous fashion sense and highly likeable personality would surely help her casting to bring even more audience members to watch the film, something that is always a good thing.