Matthew Vaughn's "X-Men: First Class" not only brought a deader-than-dead franchise back to life, it did so by making the best one yet. So a sequel was kind of expected. What wasn't expected? That Fox would go ahead and quietly register the new film's prospective title with MPAA. But they did, and AICN, HitFix and multiple others have confirmed that that title is...
...X-MEN: DAYS OF FUTURE PAST.
The internet is now broken.
Quick-version, because I'm working at the moment: "DOFP" was a landmark story-arc during Chris Claremont's defining epic run on the X-Men comics. The basic idea? It's "Terminator" (which it technically pre-dates) but with The X-Men; involving a dystopian near-future where the human government is successfully carrying out that whole Mutant Holocaust thing that "bad guy" Magneto kept warning everyone would happen. In the original version, the older future-version of Kitty Pryde zaps her consciousness back in time into her younger self and tries to prevent The Brotherhood from carrying out a political-assassination (Senator Kelly, from the first "X-Men" movie) which kicks-off the anti-Mutant backlash.
Other than Fox apparently picking this title, nothing else is known at this time... though, obviously, the immediate speculation is that a time-travel story might allow the "21st Century X-Men" of the original trilogy to share the screen (and maybe patch some plot-holes) alongside the Cold War era X-Men of "First Class." Also, it's worth remembering that DOFP's future anti-Mutant infrastucture was based around - yes - The Sentinels.