One of the teleporting mutants, Blink might fit in where Nightcrawler could have. Perhaps – just perhaps – James McAvoy did see Nightcrawler in his script, perhaps there was a replacement.
Though probably not.
We don’t know if Blink will be part of the “past” or “present” timelines. The Nightcrawler theory would put her in the “present” but there does seem to be a lot more spaces in the “past” to be filled. Where have the First Class all gone?