Well here's an interesting twist.  Bryan Singer has tweeted that Ian McKellen and Patrick Stewart have joined X-Men: Days of Future Past, which is supposedly a sequel not to X-Men: The Last Stand, but X-Men: First Class.  So how will they be used?  There are a couple of possibilities I can think of off the top of my head.  First, they'll just be a couple of cameos used as a framing device, so that Professor X and Magneto are looking back at the events of the new movie.  When these remembrances would occur in terms of the original trilogy is a bit fuzzy since Last Stand ends on the borderline of the status quo [spoilers] (Magneto loses his powers, but not really; Professor X dies, but not really).

Hit the jump for more speculation.  Singer also tweeted that James McAvoy, Michael Fassbender, Jennifer Lawrence, and Nicholas Hoult will all return.  No mention on First Class' other cast members.  Filming on the upcoming sequel is set to begin early next year.

xmen-days-of-future-past

For those unfamiliar with X-Men comic book lore, "Days of Future Past" is an arc where the characters live in the dystopian future of 2013 (the horror!), Sentinels roam the Earth, and the mutant population is being decimated.  Is it possible that Magneto and Professor X live in a dystopian future (they got better!) and the movie crosses between two time periods?  It could be a more challenging and possibly more rewarding storytelling device than simply having two characters look back on their lives.

Either way, McKellen and Stewart are welcome additions to the cast so long as there are no CGI facelifts.

Here's the official synopsis for Chris Claremont and John Byrne's X-Men: Days of Future Past:

It was the best of times, it was the worst of times: Relive the legendary first journey into the dystopian future of 2013 - where Sentinels stalk the Earth, and the X-Men are humanity's only hope...until they die! Also featuring the first appearance of Alpha Flight, the return of the Wendigo, the history of the X-Men from Cyclops himself...and a demon for Christmas!? Collecting UNCANNY X-MEN (1963) #138-143 and X-MEN ANNUAL #4. [Amazon]