3. The Thwarted Assassination
I briefly touched upon this in the last point, but the entire story of the comic book version of Days of Future Past revolves around Kitty Pryde being sent back in time to prevent the assassination of Senator Robert Kelly at the hands of the Brotherhood of Evil Mutants. This assassination triggered an uprising against mutantkind and super-powered beings in general which, ultimately, culminated in the world of 2013 (at least in the story) being dominated by mutant-hunting robot Sentinels whose intention is to end the "mutant threat" completely. In the movie X-Men universe, however, Senator Kelly is already dead - killed by a failed attempt on the part of Magneto to turn him in to a mutant himself - so he would have to be replaced. Perhaps, in the movie, someone else with high political standing from real world history could be substituted for Kelly (given that the X-Men movies tend to enjoy referring to real life historical events with a twist - the Cuban Missile Crisis in X-Men: First Class and the various wars Logan and Creed fought in being depicted in X-Men Origins: Wolverine, for example). Matthew Vaughn, who was one of the writers for the film, has actually gone on record saying that he'd like to portray the assassination of JFK in an X-Men movie, having Magneto be the perpetrator (and who better to produce an impossible-to-trace bullet than Magneto?). Of course, in the comics it is Mystique, leading the Brotherhood of Evil Mutants, who is out to kill Senator Kelly - and as we all know, she has sided with Magneto by the end of X-Men: First Class.