So, comic-related spoilers ahead. You've been warned.
In the classic X-Men: Days of Future Past storyline from Chris Claremont, Kitty Pryde travels back from a dystopic future to... eh, here's the summary from Wikipedia:
The storyline alternates between present day, in which the X-Men fight Mystique's Brotherhood of Evil Mutants, and a future timeline caused by the X-Men's failure to prevent the Brotherhood from assassinating Senator Robert Kelly. In this future universe, Sentinels rule the United States, and mutants live in internment camps. The present-day X-Men are forewarned of the possible future by a future version of their teammate Kitty Pryde, whose mind traveled back in time and possessed her younger self to warn the X-Men. She succeeds in her mission and returns to the future, but despite her success, the future timeline still exists as an alternative timeline rather than as the actual future.my review of that film is here) established young Xavier played by James McAvoy and Magneto played Michael Fassbender, along with (new Oscar winner) Jennifer Lawrence as Mystique. It's likely they'll be the focal points of the story, but Ellen Page is set to return as Kitty Pryde. Good news is that the original Magneto and Professor X are also set to return, as Ian McKellen and Patrick Stewart have both signed on, so time jumping is definitely going to happen. This means we'll be seeing a bunch of actors from the original movies.
What is interesting is the potential this storyline has to effectively erase the original three (or four if you include X-Men Origins: Wolverine) X-Men films and start over, sort of like JJ Abrams' Star Trek.
Meanwhile, in the most awesome piece of news you are ever going to hear, Game of Thrones star Peter Dinklage has singed on to play an as-yet-unnamed villain. Who's he playing? I couldn't care less. It's Dinkage, people!
So that's everything that's in so far. Rumor has it filming starts in May.