From CinemaBlend:
Bryan Singer, who has agreed to return to the X-Men franchise to helm Days after Matthew Vaughn walked away, took to Twitter to share that Stewart and Sir Ian McKellen would be in the planned film, reprising their roles as Prof. X and Magneto.
Of course, this doesn’t saw whether they will be cameos or have any real depth. Hugh Jackman had a cameo in First Class so a similar instance may occur in Days of Future Past, likely when whoever goes back in time goes back in time. (I still don’t think it will be Kitty, ages don’t fit).
Now, there is also the little issue of Xavier being dead, maybe, not really sure, depends on how you want to read the Easter Egg on Last Stand (plus it’s not like he hasn’t died and come back before, canon wise). But then of course Jean is also dead and apparently she’s making a cameo in The Wolverine (which since it hasn’t been categorically denied by now lends to the very likelihood that a cameo was shot).
This news is another piece that leaves me feeling neutral. Either this could make things that much better or that much worse… all depends on how it plays out.
X-Men: The (fan fic) Series 