Re: So what will be this Preventable Event?
Concerning the Kennedy Assassination, maybe it would work if they have to make it happen instead of prevent it from happening because letting Kennedy live would cause major negative ripples in the future time stream. I'd see that as an interesting way to explore a conflict of conscience within Charles having to let Magneto manipulate the bullet to kill Kennedy.
"There are no strings on me." - Ultron, Avengers 2: Age Of Ultron