Remember X-Men Origins: Wolverine? What a disaster.
The first solo character spinoff of the lucrative X-Men film trilogy threw Hugh Jackman's beloved Wolverine even more front and center than he already was and forced in a pair of fan-favorite Marvel Comics characters in supporting roles. Those two characters were Deadpool and Gambit, and both were nearly ruined. Thankfully, time heals all wounds.
If the reviews weren't bad enough for Origins (it's the worst reviewed installment in the franchise), it almost killed the brand too. Wolverine ended up getting another chance with James Mangold's The Wolverine, completely ignoring its predecessor, and the core X-Men series avoided any connections to the spinoff as well when it quasi-rebooted with Matthew Vaughn's X-Men: First Class. But the series is still going and Wade Wilson a.k.a. Deadpool a.k.a. the Merc with a Mouth is back with Ryan Reynolds playing him (again). How is that possible?
Time-Travel, The Ultimate Plot Fixer
Fans can be as thankful as original X-Men director Bryan Singer that the creative and producer team of Matthew Vaughn, Simon Kinberg, and Lauren Shuler Donner pitched the idea to finally bring in time-travel to the X-Men movie universe with the sequel to First Class. Time-travel is commonplace in the X-Men comics but in Hollywood, it's surprisingly not been explored in the big comic book cinematic universes. That is, until 2014's X-Men: Days of Future Past.
X-Men: Days of Future Past sees mutant freedom fighters in a dystopian future send Wolverine back in time to the '70s to alter the course of history and change the future, to a better "happy ending." This version of the '70s was already different than continuity explored in the original X-Men trilogy and its spinoffs, and now it's far more different thanks to the X-Men saving the day in 1973. If that's confusing, don't worry. It's practically meant to be in an effort to emphasize the point that every X-Men movie can exist in the same continuity by explaining plot holes and differences away with the concept of alternate timelines.
As we explored previously in an article about our visit last summer to the set of X-Men: Apocalypse, Bryan Singer has found a way to do anything he wants with future X-Men movies. Essentially, the franchise can be rebooted infinitely, but still be considered canon. But for now, the plan in place is for a singular continuity going forward that takes place in the reset world after the 1973 events of X-Men: Days of Future Past. This universe, where Magneto dropped a stadium around the White House, where mutants are well-known, and where Apocalypse will lay waste to parts of the world a decade later, is the same universe where Deadpool, Gambit, and Wolverine 3 take place later in the timeline. Here's Simon Kinberg's explanation:
"[X-Men: Apocalypse] takes space chronologically before those other films, so it’s more like those films have to acknowledge this than we acknowledge Gambit, Deadpool, or Fantastic Four or anything else that exists within the sort of Fox/Marvel universe. But I work on all of those films in one capacity or another, either as a producer on all of them and as a writer on Fantastic Four and this movie, so I’m certainly aware of all the different stories we’re telling at the same time, and they all are part of a larger fabric now, and so the world of Deadpool, the world of Gambit exists in a post-Days of Future Past post-Apocalypse world where all of these stories are the same as our shared history. The same way that each of us of different ages knows about Nixon and knows about Reagan and knows about 9/11, our fictitious events like the stadium dropping on the White House in 1973 is part of the world in which Gambit, Deadpool, Wolverine on forward exists."
Bryan Singer adds, "I rebooted the universe so now anything can happen," with the caveat that their theory of time-travel and alternate universes revolve around immutability, in that things may differ, but key events in every timeline will remain mostly similar. Xavier will always form the X-Men, Wolverine will always enlist in Weapon X and get his adamantium claws, and Wade Wilson will always become a version of Deadpool.