A sequel to Matthew Vaughan's X-Men: First Class is pencilled for release on July 18th 2014 and set to starting shooting next January. Since there are more than two years until the movie hits cinemas, not many details have been released, but AICN are reporting that Fox have registered the name Days Of Future Past with the MPAA. This was a two-issue story back in 1981, with mutants from an alternate future travelling back to the past to prevent the assassination of a Senator by Magneto's Brotherhood, the outcome of which was the emprisoning of all mutants in internment camps and classic X-Men villains, Sentinels, in control of the United States.
Bearing in mind that this claim is unsourced and thus firmly in rumour territory, but the storyline raises all sorts of possibilities for melding the young First Class cast with those from the Bryan Singer X-Men movies. The alternate timeline plot would also bring something a little different and more complex at a time when comic adaptations are increasingly recycling a handful of tired storylines. It's likely to be a while before any of this is denied or made official, but hopefully there's a glimmer of truth to it.