Comic book fans get your geek on, Bryan Singer Producer of "X-Men: First Class" has confirmed that the storyline for the sequel will be based on the very popular story arch "Days of Future Past" from the original saga of comics. The "Days of Future Past" storyline involves the 1980's X-Men team.
"In a video sitdown with IGN, Singer says that the movie —
currently being written, and set to start shooting “in a few months” —
will be called Days of Future Past, which is also the title of one of
the most memorable X-Men story arcs ever."