With time travel or alternate dimensions, the door is open for nearly anyone from the first 3 X-Men films to may a return in Days of Future Past. That's why actor Alan Cumming, Kurt Wagner (Nightcrawler) in the second X-Men film was asked about a possible return to the franchise.
"I haven’t gotten the call. Someone told me that Nightcrawler doesn’t appear in the story but I don’t know, I think they would have called by now. It was funny that film because I really like it and everyone really responded to Nightcrawler and enough time has elapsed that I would like to go back to it. Although then when I did that film it was sort of the start of this spate of superhero and comic books made into films so it felt a little more special then it does now. X2 was a really great film, not just as a comic book film. I think it’s one of the films I’d been in that I think of as really good."
That would make for an interesting story seeing that Mystique is his mother and Azazel is his father. There was a cut scene during the beach fight in X-Men: First Class where Azazel shouts "You will have my baby." But I'd probably say he won't be back, cameos withstanding seeing as how that's a very complicated storyline for a film that already has very busy moving parts.
Singer has already started preproduction on the film and reports speculate that filming could commence sometime in Spring 2013.
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