This sounds like a great idea for a movie. Not only can they use it to firmly give First Class its own timeline (ala the Star Trek reboot), it also gives us the opportunity to see Michael Fassbender, Jennifer Lawrence, Hugh Jackman, Ian McKellen and Ellen Page in one movie. A group of A-list actors that rivals the Avengers. And it can give us more of the underdeveloped characters from X3 like Kitty, Colossus and Iceman.
Here's a Wiki page for the original story: http://en.wikipedia.org/wiki/Days_of_Future_Past