Nightcrawler was a great character, but there needs to be room made for new characters. X-Men 2 is one of my favourite movies because it managed to give a dozen or so characters solid, interwoven stories. But there is a limit to how many stories you can tell in a movie. Keeping all the characters from X-Men 2 and adding Beast, Juggernaut, Angel and a new villain, plus expanded roles for Shadowcat and Storm, plus Wolverine's story, plus Dark Phoenix, plus whatever else they chuck in is already a large chunk of movie.