Hey, it's been a while! This is a character design I've been meaning to do for so long. It actually turned out to be one of the most challenging so far.
I wanted to move away from the design direction Mystique has been taken in the previous movies and stay closer to her original comicbook look. I stayed away from the temptation of updating her costume to a combat uniform like Black Widow and Captain Marvel wear and stayed closer to her traditional white dress from the comics, with a few updates and additions. I think of Mystique as a femme fatale archetype, yes she can kick some serious ass, but she relies more on her cunning, and mutant ability rather than brute force. So there's less need for protective clothing, a dress fits in more with those character traits. You would think a white dress would be easier to design but sometimes the simpler the design the harder it is to come up with something cool, especially when trying to retain the look of the original.
I Had lots of fun trying to tie in design themes for her dress to her character. I played around with lots of different materials, patterns, colours, shape design to represent Mystique's shape-shifting ability. I eventually created a custom pattern that transitions from squares to dots, and gave it a subtle iridescent colour to tie in with the changeable theme. It's the little details in character design I love to do.
As for an Actress I think would kill it as Raven Darkholme - Charlize Theron. I think she would be perfect. I can totally imagine her nailing that matriarch type role for Rogue in the Evil Brotherhood.