Even though X-Men: Days of Future Past combined the casts of the original trilogy and X-Men: First Class, certain characters had much more importance than others to the story. Most notable was Mystique, who was not only responsible for creating the X-Men’s dystopian future, but was also the source of the future Sentinels’ power mimicry abilities. However, if X-Men: First Class director Matthew Vaughn had stayed on the film rather than passing to work on Kingsman: The Secret Service, Mystique’s role would have been even bigger.
While chatting with Collider, Vaughn mentioned that while 90% of his original X-Men: Days of Future Past story made it into the final product, there were still a few changes made, among them the idea that the future Sentinels would actually look like the notorious blue shapeshifter. Said Vaughn,
"My idea was the sentinels at the end, I wanted them to look like Mystique. I thought there should be thousands of mystiques attacking them in the future."
Instead, Singer and company decided to go in this direction:
While the 1970s Sentinels in the film looked similar to their comic book incarnations, the future Sentinels evolved to a point where they looked almost organic, with their shape-shifting abilities and scaly exteriors. Would making them all Mystique lookalikes have made much of a difference? In anything, it might have made that final scene where the Sentinels are annihilating all the X-Men less cool-looking. Still, as with many What-If? scenarios in the movie industry, it might have been interesting to see Vaughn’s entire vision for X-Men: Days of Future Past on the big screen (this version also had Juggernaut breaking into the Pentagon instead of Quicksilver).
In the main timeline of X-Men: Days of Future Past, which is to say what led to the future apocalypse, Mystique killed Bolivar Trask in revenge for the brutal experiments he conducted on mutants. This set into motion the events that would lead to most mutants (and humans) being wiped out by the Sentinels. The mutant-killing machines were upgraded with Mystique’s DNA, allowing them to adapt and use mutant powers. With the new timeline in effect though, mutants have nothing to worry about from the Sentinels. Since Mystique spared Trask’s life and stopped Magneto from killing President Nixon, the Sentinel project was been shut down, and everything is all fine and dandy in the future. She went from being the person who unwittingly causes the end of the world to someone who helped improve public opinion of mutants.That’s quite the turnaround.
Jennifer Lawrence will reprise her role as Mystique in 2016’s X-Men: Apocalypse, and if recent reports are any indication, her role may even be bigger than it was in X-Men: Days of Future Past. We'll be sure to hear much more about it as we get closer to the start of production next year.