Re: So what will be this Preventable Event?
Though X3 put a rosy spin on the fallout from "The Last Stand", with the Golden Gate bridge being rebuilt and Erik playing chess in the park, it is reasonable to assume that the high profile nature of Magneto's attack on the island and the near extinction level event created by a former well known mutant advocate would have dire consequences. I could certainly see fearful voters pressing their political representatives to deal with the mutant situation by any means necessary.
The cause of much of this destruction, and the murders of both a preparatory school dean and professor, was Jean Grey. The split in her personality created the third movie's Phoenix, with all of Jean's power and none of her conscience. I thought one of the best scenes in the third film is the confrontation between Logan and Charles in the examination room, in which Professor X takes off the saintly robes and cruelly attacks Logan, refusing to admit his horrific mistake.
I would love to see the First Class Sequel rectify this, with a few survivors (Jackman, Berry and Grammer reprising their roles) running from Sentinels to find the Cerebro created in First Class and attempting (through science!) to send a message to Charles or to their younger selves. This could run currently with a 60s era plot in which a young Jean Grey proves to be a threat, with Erik and Charles debating whether how they should deal with her powers. The message from the future would be that Erik, in this case, was right.