Re: So what will be this Preventable Event?
If they are daring enough, it could be the President Kennedy assassination with Magneto controlling the bullet. Vaughn suggested this in interviews.
If that proves too contentious, the assassination of a fictional figure perhaps?
Batman v Superman 6/10, Captain America: Civil War 7.5/10, Deadpool 8/10, X-Men: Apocalypse 8/10, X-Men: Days of Future Past 9/10