What if the event is the birth of Jean Grey? Sort of repetitive in that John Connor mode, but think about it. A mutant more powerful than Xavier and Magneto? Albeit how would anybody know that of an infant? The event ensuing from the end of TLS is the creation of the Sentinels. Maybe if she is killed off at birth, the war against mutants would never escalate to the point where Sentinels are required. But given the premise of Nixon and the Washington D.C backdrop movie has going for it, it will probably be politically motivated.