In my opinion, Spider-Man has the best supervillains in the Marvel Cinematic Universe--so my top choice would have to be Black Cat, whom I believe would be an excellent role for Kristen, for here we have a layered character who blurs the lines between good and evil, someone who can be both hero and villain. Such a character would not only be a challenging part, if done right it would ring more true than in the case of most comic book villains, as few people in real life are the epitome of sainthood or pure evil--more of a mixed bag if you ask me.
Another choice would be if creators of the next installment of the Captain America franchise decide to use the villainess Sinthea, I believe Kristen could be a welcome addition here also. However, after the departure of Red Skull from this franchise, I'm not sure if it would make a whole lot of sense to introduce Sin who is the daughter of Red Skull. Hugo Weaving who played Red Skull mentioned he's not interested in reprising the role--so perhaps they can cast him in a cameo appearance, just long enough to introduce Sin to the MCU.
I could Kristen as Spider Woman. In BD pt. 2 she did show many similarities to Spider Woman. The "shield " , the super strength, the detective skills used when attempting to locate Alice and yes her intimacy with Edward. And Spider Woman is VERY intimate super hero. Also, Kristen's English accent as Snow White and Jessica Drew/ Spider Woman is British. But then again, Scarlett Johansson plays Natasha Romanoff/ Black Widow, who is supposed to be Russian but Scarlett Jo just speaks with her normal American accent. So perhaps Kristen can do the same.
I think if the character of a Marvel or DC story had a good mix of temperament and personality, a challenging character type, then it could appeal to Kristen. And agreed that it would be something that fits among the types of characters she portrays. Good ideas here. It'll be interesting to see if she takes on one of the super villain/hero types in future.