7. Jean Grey
Some might feel that Dr. Jean Grey's character is a little played out by now - Famke Janssen played the role superbly in the first three X-Men films, leading to a fiery climax - but I personally would be very glad to see her show up in Days of Future Past. Given how First Class foreshadowed warfare between his own side and Xavier's, it follows that Magneto would want to recruit the most adept mutant soldiers possible, and who has more terrifying potential than Jean Grey? What was most interesting to me about First Class was seeing how the relationships we took for granted in X-Men developed, particularly that been Xavier and Magneto; throwing Jean Grey into the mix has a wealth of dramatic potential indeed. As long as the casting works, seeing a young Jean Grey making acquaintance with Magneto and taking us through the complexities of their relationship could be cinematic dynamite.