Ian McKellen, Patrick Stewart coming back for "X-Men: Days of Future Past"

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What's better than an X-Men movie starring James McAvoy and Michael Fassbender, as Professor X and Magneto?

How about one that also stars the two giants of stage and screen that played the original film versions of those roles, Patrick Stewart and Ian McKellan?

In a casting move that can only thrill franchise fans, all four actors are set to return to those iconic roles in the upcoming X-Men: Days of Future Past, headed for theaters in July 2014.

Producer and Director Bryan Singer, who also helmed 2009's X-Men: First Class, tweeted out the big news Tuesday evening.

I'd like to officially welcome back James McAvoy, Jennifer Lawrence, Michael Fassbender, & Nicholas Hoult to #XMEN for #DaysOfFuturePast

— Bryan Singer (@BryanSinger) November 27, 2012

Thrilled to announce @ianmckellen118 & @sirpatstew are joining the cast of #XMEN #DaysOfFuturePast #magneto #professorX More to come...

— Bryan Singer (@BryanSinger) November 27, 2012

So one wonders what other time-and-space bending casting decisions might come next. Since Jennifer Lawrence (Raven/Mystique) has definitely signed on, could we also see Rebecca Romijn return to the role? Hank McCoy/The Beast was portrayed by Kelsey Grammar in X-Men: The Last Stand, so would he return to stand side-by-side with Nicholas Hoult?

And furthermore, will the future and past (see what I did there?) versions of the characters appear simultaneously, or exist only in flashbacks or flash-forwards? Will both older and younger Magneto and X have substantial screen time, or will they use McKellan and Stewart in a way similar to 2009's Star Trek employed its Leonard Nimoy cameo?

No matter what the Days of Future Past team decides, it's bound to bring in the X-Men fans of all generations.