Re: So what will be this Preventable Event?
Why not tie it into X3: The Last Stand and make the preventable event be the discovery of the "cure?" Warren Wirthington II is the man behind the cure in the third film. So in Days of Futures Past have Kitty (or Rogue) travel back in time to warn the X-Men about the cure.
If you have Rogue go back, you can have someone important die post-cure that would have been someone Rogue could have saved had she not been cured. Or, you can have Kitty go back to warn them about the war on Alcatratz Island and the death of all of those mutants that it caused... Jean, Scott and Xavier amongst them.
So it's the job of the past X-Men to hunt down and find a young Warren Worthington II and change his mind, and get him to accept mutants.
Worthington II never creates the cure.... X3:The Last Stand never happens. Boom.