There's a lot going on in the X-Men film franchise. We got one trilogy that ended on a terrible, fan-souring note, then a second series of movies focusing on the main character of the original trilogy, then a reboot that recast all of the main characters as younger versions of themselves in the 1960s, and now a sequel to that reboot that combines the original cast with the reboot cast while simultaneously confirming that all seven movies are actually part of the same continuity and buying back everything that happened in the Brett Ratner penisburp that was X-Men: The Last Stand.
While Days of Future Past does a good job of connecting all those disparate dots, there are still a fair amount of plot threads left dangling in the wind, along with a litany of unanswered questions and things that are straight up presented to us but never explained (such as "Why the hell are all the files in Peter Dinklage's office kept in a vertical cabinet 5 feet off the ground?"). But what's most disturbing are the storylines implied by the movies, which manage to be deeply unsettling without ever getting a minute of screen time.
4 Professor X Stole a Young Man's Body and Mutated Him into an Exact Physical Double of Himself (And Is Secretly Addicted to Drugs)
At the end of X-Men: The Last Stand, we see that Professor X has somehow managed to transfer his consciousness into the body of a brain-dead coma patient. So even though we saw him get ripped apart by Jean Grey's series-destroying celestial black magic, we at least got a little relief in knowing that he would live on, albeit in the body of some guy whose family was about to receive some challenging news.
20th Century Fox
"So our family gets to upgrade from Dale to Patrick Stewart? Sweet, fuck Dale."