Re: So what will be this Preventable Event?
What I don't want to see is the First-class timeline's Wolverine joining the team. We already got a newby Wolverine joining the team in X1.
I want DOFP to show how a young Scott, Jean, Storm (who end up being born in the 40/50's) join the team.
Either keep First class timeline's Wolverine out of it (perhaps time-traveller Wolverine sets the other Wolverine on a different path, away from the X-Men), or have mind-travel, like it was in the comics (original timeline's Wolverine's mind takes the place of First-class Wolverine's mind).