Titled Just "Dark Phoenix" for PC Social Justice Reasons?
It appears from the trailer, the wikipedia page, and other sources I've seen that this movie will simply be promoted as Dark Phoenix and not "X-Men: Dark Phoenix", which is breaking away from the trend of the past several X Men movies (not counting the Wolverine spin-offs)
i can't see any reason for doing this other than to be politically correct. The promotion has made a big hoopla about how this will be a very woman-centered movie, with Sophie Turner, Jennifer Lawrence, and Jessica Chastain having the main roles.
So considering that, I'm sure some SJW feminazi blogger would have conjured up a controversy about how the movie is still called X "MEN" even when it's supposed to be about women. And then some weirdo on Twitter will propose permanently changing the canon so that the name X Men will change to something gender neutral
It may sound farfetched, but I think this is the case. This is the world we live in today