Margot is not going to be Emma. X-Men are still 3-5 yrs out, and I doubt Emma would be on the leading edge. It's unfortunate its going to take so long, the MCU just debuted the perfect Emma actress, as freaking CLEA?! What a waste. I'm sorry, but I don't like Benedict's Strange, and Clea is a waste of Charlize, who would have been the absolute perfect Emma! Margot as Sue would be an inspired choice, certainly injecting a somewhat bland archetype with a more pop culturally relevant actress. Inspired and modern, but maybe too good to be true? Celebrity gossip and fandoms are treacherous waters. We'll just have to see what actually is true and what is just clickbait.
I do wonder what age they will go with for the main cast though. A lot of the Avengers are being replaced with children/teens/young adults. I always thought the MCU should do a proper long-form storytelling of the X-Men by starting with teens/college aged young adults and show us them develop individually and as a team over the course of 10-20 years, finally maturing to the statesmen and galactic leaders we see them now as in the current comics. I think it would be a great mistake to start them off as 30/40 year olds in Krakoa, for example, given how deep and rich their canon is, and how voracious Disney's appetite for content is(you could make 5000 D+ shows just with X-stories/characters). Feige has so much beloved canon to adapt, I hope he takes his time and doesn't try to cram too much in too fast. The fact that they've had the rights back for so long and have barely touched them yet makes me feel like they are going to take their time and exploit the hell out of the rich IP. But again, only time will tell.