It's completely moot considering Gwen is already cast and most are quite pleased with the casting. It's not even a matter of who she would be better fit for, but whether she would fit in this particular role. If you remember, it wasn't that too long ago where most were convinced if Stone were cast that she'd be MJ. Lo and behold, she's in the polar opposite role -- and pulls it off wonderfully.
I would agree with you, even your Fox example (pre-TF2 days anyway), but you're underestimating the magic stylists and make-up artists can do. A lot of these young Hollywood girls could be made to match up with Fox in the sexpot category if they were actually going for that appearance. Fox has just made a habit of making that her image. If there's one thing I've learned greatly about women's looks, it's that hotness is a byproduct of clothes, attitude, and make-up. Take away all that (i.e. completely natural) and you're more likely to consider them cute and beautiful, more than sexy or hot. If you've seen candid pics of a natural Fox, especially before her fame, you'll see this very clearly.
So in my opinion we need someone who's at least 7 or an 8... naturally. Hollywood will easily stroll her to that extra 2 points if it's needed. Since I'm currently on the Poots bandwagon, I ventured to scour for pics of her that capture Imogen in a very different light than most people are familiar with. Even though I personally think these shots show her potential to be a very sexy MJ, this is more of an example of what glamouring up can do to raise a female's physical appeal.
*base image for her best shot as she usually looks*
*glamoured up*
A classy aesthetic, but imagining her looking like that as a redhead and with more modern clothing -- it's the closest I've seen (thus far) from a young and capable actress, to looking like how I could see a supermodel MJ in live-action.
