9. Rachel Summers
If you thought that Kitty Pryde’s time-manipulating abilities in Days Of Future Past didn’t make any sense for her character, that’s because they were never intended for her. Instead, the mutant responsible for sending Logan back in time was originally meant to be Rachel Summers, the child of alternate future versions of Cyclops and Jean Grey.
Rachel Summers was clearly designed for this function in the film, and writer Simon Kinberg even admitted as much. He explained that despite her complex emotional role and natural fit, her complicated origin story was too much to ultimately justify including.
It's clear how essential she was to these initial drafts, as amongst her telepathic powers is the ability to astrally project herself back into the past, something that Shadowcat seemed to spontaneously manifest. Another new inclusion in the film might have been pushing it but surely Rachel would have made more sense than the explanation we got.
As a result of her removal from the script, Rachel Summers remains one of the most prominent X-Men to have not appeared in the films yet. In particular, she has deep ties to the Phoenix Force, so should the new X-Men films revisit that storyline, it could be a great opportunity to finally introduce her.